An expanding Cuban spy network has infiltrated Miami's exile community, used the Cuban UN mission as headquarters for espionage in this country and placed its global operations at the service of Soviet intelligence needs, according to the highest-ranking Cuban agent known to have defected to the United States.

These and other inner workings of Fidel Castro's intelligence service are detailed in a book to be released next month by E. A. Seemann Publishing Inc., of Miami.

Orlando Castro Hidalgo, author of the book, "Spy for Fidel," was a top agent in Paris for Cuba's Direccion General de Inteligencia, or DGI.

Disillusioned with the Cuban regime, he and his family fled to Luxembourg in April, 1969, taking secret files with them, and obtaining U.S. asylum.

His defection became publicly known months later, and so did some of what he told U.S. officials. But he now gives a fuller picture of Cuban intelligence and its role in furthering Cuba's revolutionary designs.

Among the items on his personal "scoreboard," he claims to have named more than 150 agents and contacts of Cuban intelligence and revealed "the extent to which intelligence has taken over the Cuban diplomatic service, as well as other governmental organizations."

Castro Hidalgo outlines Cuban efforts to spy on exile organizations in Miami. He says one key agent, believed responsible for the capture of a number of exiles invading Cuba, still lives in Miami.

"Because his activities have not directly involved espionage against the United States, charges have not been brought against him," the ex-agent relates.

Castro Hidalgo says Cuba has never had trouble infiltrating agents into the United States. He mentions available channels such as the refugee airlift, small-boat escapes, fence-jumping into the Guantanamo naval base, and emigrants who travel via Spain or Mexico.

But well-laid plans did not always work, he claims. In 1965, when Cuba let exiles leave in small-boat flotillas, "a DGI official began recruiting large numbers of the refugees to work for the intelligence service once they arrived north.

"Rather than endanger their hopes of leaving, the refugees agreed to the official's demands. Some of them evidently took seriously their promise to help: DGI began receiving reports from a number of them.

"The official had failed, however, to instruct his recruits on how to identify themselves when sending their reports, and when these began arriving, DGI was unable to determine who was sending them. The reports were virtually useless."

Because Cuba has no diplomatic mission accredited to the United States, Castro Hidalgo says, its intelligence center at the United Nations "serves as headquarters for Cuban-directed subversive and espionage activities in the States." He notes that a number of Cuban UN officials have been expelled by the United States for espionage work.

Before going to Paris, he relates, he worked for DGI a "counterrevolution unit" in Havana concerned with "infiltrating and spying on exile organizations operating out of Miami."

He says he was given dossiers on a number of key exile figures, including Alpha 66 leader Andres Nazario Sargen. "My task was to build up information about these persons with the view of finding a way to place a DGI informer close to them," he says. He does not indicate if the plans worked in Nazario Sargen's case.

The defector writes that a key function of the Cuban intelligence operation in Paris was to facilitate the travel of Latin American revolutionaries to and from Havana by way of France and Eastern Europe. It also provided courier service for messages from Che Guevara to Cuba when the late guerrilla leader was trying to foment revolution in Bolivia in 1967.

Castro Hidalgo says that the DGI has "virtually become an arm of Soviet intelligence," and that the DGI has all but taken over the Cuban news agency, Prensa Latina, and the Cuban Institute for Friendship with the Peoples (ICAP).

The diplomatic service, too, is preponderantly DGI, he says, and in Paris, "Virtually the entire staff - except the ambassador and the counselor - were intelligence personnel."

Castro Hidalgo, who fought in Fidel Castro's rebel army and against the Bay of Pigs invasion says he is pursuing a new life in the United States under another identity. He says it is in "suburbia"and is not in Washington, but he doesn't say where he now lives. He will not fly in airplanes, he comments, because they can be hijacked to Cuba.