WASHINGTON - The State Department Wednesday expelled the second-ranking Cuban diplomat in the United States for engaging in "intelligence-gathering activities" and conspiring with American businessmen to violate the embargo on U.S. trade with Cuba.

Ricardo Escartin, first secretary in the Cuban Interests Section in Washington, was given a week to leave the country. He is the first Cuban diplomat to be expelled from the United States since 1970, when two members of the Cuban mission at the United Nations were forced to leave.

Escartin, according to State Department spokesman William Dyess, had traveled extensively in the United States in the past two years, making contacts with American businessmen interested in trading with Cuba despite the 20-year-old trade embargo.

It is believed that at least some of the businesses involved are based in South Florida.

In fact, the U.S. attorney's office in Miami said Wednesday that it, too, is investigating numerous individuals and businesses in South Florida for possible violations of the "Trading With the Enemy Act."

At this point, there is no direct link between the Washington and Miami investigations except that both involve allegations of illegal commerce between American businesses and Cuba.

One source close to the Miami investigation said, "Beyond a doubt, it [the illegal trade] is going on right now. People in the Cuban government are using their connections here to get a generator or tires or whatever."

The Miami probe originated with last year's Mariel-to-Key West boatlift.

Under the "Trading With the Enemy Act," commerce with Cuba is forbidden unless specifically authorized by the Treasury Department's Office of Foreign Assets Control. Last May, the office made virtually all transactions related to the boatlift violations of the act.

The State Department said that, in the investigation of Escartin, it learned that many of the American businessmen contacted by the diplomat subsequently visited Cuba and arranged to engage in trade through dummy corporations controlled by the Cuban government but located in other countries.

"Were it not for his diplomatic immunity, Mr. Escartin could be prosecuted" for conspiring to violate the trade ban, spokesman Dyess said.

He said that some of the American businessmen apparently didn't know that the products they sold to these dummy corporations were destined for Cuba. But, he added, "Unfortunately, some U.S. entities knowingly collaborated with Cuba in this illegal trade."

The State Department acknowledged that indictments are possible in the Washington investigation, which is being conducted by the FBI, the U.S. Customs Service and the Treasury Department.

Escartin's activities were monitored by the FBI, which concluded that the diplomat was "an intelligence agent" for Cuba. No details were released to explain the espionage allegation.

The Cuban Foreign Ministry, in a statement issued through the Interests Section, flatly reject the "false accusations" of the State Department.

It said the expulsion order "reflects the intention of U.S. authorities to continue the policy of hostility toward Cuba and of antagonism toward its representatives in the United States."

The United States imposed the trade embargo on Cuba in October 1960 in an effort to undermine the government of its then-newly installed premier, Fidel Castro.

The embargo was partially relaxed in 1975 - primarily because of pressure from powerful American businesses - to allow overseas subsidiaries of U.S. companies to trade consumer products with Cuba as long as the goods weren't manufactured in the United States.

The result has been that most multinational corporations have gotten around the embargo, according to critics of the policy.

Sources in Washington said Escartin's role was "childishly simple." He would travel around the country contacting businessmen and trying to persuade them to violate the embargo by selling their products to Cuban-controlled fronts in third countries, the sources said.

These dummy corporations have been identified publicly by the Treasury Department as Cuban agents, thus putting American businesses on notice that trading with them would be a criminal violation of the embargo. Such offenses are punishable by up to a $10,000 fine and 10 years in prison.

The Cuban agents' list includes seven firms and three individuals in Canada, Panama, Jamaica, and Czechoslovakia. Among companies involved directly with Escartin, sources said, are Moonex International of Kingston, Jamaica, and CIMEX, Servimpex, Servinaves, Leybda Corp. and Havanatur, all of Panama.

A Hialeah-based firm, Travel Services Inc., also is listed as a Cuban agent. But Travel Services, which arranges tourist trips to Cuba, is licensed by the State Department to do business with the Cuban government.

The sources said the American firms engaged in the illegal trade included small and medium-sized companies that sell such goods as small appliances, auto parts, machinery, tires, batteries and record players. Those goods are hard to get in Cuba.

State Department officials said the investigation that led to Escartin's expulsion was begun several months ago by the Carter Administration.

Although the action is apt to appear as a signal form President Reagan that he will follow a tougher line against Communist diplomats, a department source said the change in administration had nothing to do with it.