The Carter administration, reversing the past U.S. practice of welcoming Cuban refugees in almost unlimited numbers, has decided that the sealift of asylum-seeking Cubans moving into Flordia must be choked off before it becomes a massive new immigration wave.

Administration sources, while conceeding that denying refuge from Cuba's communist system is certain to be politically controversial, insisted yesterday that the United States cannot at this time absorb hundreds of thousands of Cubans as it did in the 1960s.

To do so, the sources said, would raise the risk of major economic and social problems, particularly when the country is entering a period of recession and budgetary austerity that could be aggravated by the need to provide jobs, housing and services for massive numbers of new immigrants.

The sources acknowledged that the impromptu flotilla of private boats ferrying Cubans to Florida cannot be turned off completely. They concede, too, that those refugees who manage to reach Key West or other Florida ports will, in one way or another, be allowed to remain in this country.

But, the sources stressed, the administration intends to try to hold the number to a few thousand at most. To accomplish that, they said, the government hopes to shrink drastically the number of boats now engaged in the sealift by its threat to fine the owners and, if necessary, imprison them under the laws governing illegal immigration.

However, while officials here said that policy was being applied with some initial success, indications last night were that the orders being issued from Washington were not being enforced in Florida against the boat owners or those paying them to make the 180-mile Cuba round trip.

In Washington, officials said that federal enforcement agencies such as the Immigration and Naturalization Service, Coast Guard and Customs Service were directed to warn boat owners of the potential penalties and to order those known to have brought in refugees not to put to sea again until they have paid fines of $1,000 per illegal passenger.

Reports from Key West told a different story. In some instances boat owners were simply ignoring the orders of federal officials, and the officials in many cases appeared confused to process incoming and outgoing boats in routine fashion.

Some sources said that if defiance of the warnings continued, the government might be forced to start impounding boats or even arresting persons involved in the sealift.

In any case, the sources here insisted that, whatever initial problems might be involved in the implementation, the administration is determined to cut the refugee traffic between Cuba and Florida to a trickle.

That policy is in marked contrast to the way that Washington has reacted in the past to Cuban President fidel Castro's practice of defusing unrest on the island by exporting massive numbers of malcontents to the United States.

That happened in the period from 1959 through 1961, shortly after Castro's communist regime came to power, and again in 1965 and 1966. On both occasions, several hundred thousands Cubans dissatisfied with Castro's rule fled the country.

In fact, when the second wave of massive flight began in October 1965, President Lyndon B. Johnson went to New York to sign, in the shadow of the Statue of Liberty, an immigration bill opening the door to a series of "Freedom Flight" that brought approximately 260,000 Cubans to this country in the ensuing months.

However, Carter administration sources noted yesterday, that happened at a time when the 1961 Bay of Pigs incident and the 1962 Cuban missile crisis were still fresh in the minds of the American people and anti-Castro feeling was high. It also was a time of prosperity and a booming job market, the sources said.

Since then, the sources added, conditions have changed in a number of respects. The immigration laws have been made stricter and more specific at a time when there is great pressure to take in refugees from Vietnam, Cambodia, Haiti and other countries as well as Cuba.

In addition, the sources said, the depressed state of the economy and President Carter's budgetary cutbacks have severly restricted this nation's ability to provide jobs and housing for large numbers of immigrants and to integrate them into American society.

A massive influx of Cubans would pose an especially acute problem, the sources contended, because most would tend to congregate in the Miami area, which could not handle them without a massive infusion of federal financial assistance.

The sources said administration soundings indicate that this view is shared by Congress, whose cooperation would be necessary for any massive refugee resettlement program.

As a result, they added, the administration reluctantly concluded that it no longer can allow Castro to send hundreds of thousands of Cubans streaming toward the United States.

Instead, the sources said, the U.S. aim is to block Castro from this and force him to move toward a rational and organized way of dealing with Cuba's emigration problems through cooperation with the international community.

The sources concede that the administration's strategy will evoke fierce opposition in the Cuban-American community and could become an issue in this year's presidential elections. Carter's expected Republican challenger, Ronald Reagan, already has accused the president of not doing enough to help those seeking to escape from Cuban communism.

The sources were unable to say how carefully the political implications had been weighed at the White House. Some pointed out, though, that the 800,000 Cubans in this country -- 500,000 clustered around Miami -- are not a major political factor outside of Florida.

Roughly half of the Cubans in this country are not citizens of voting age, and those who are tend to vote for candidates, usually Republican, who stress a continuing hard-line, Cold War approach to dealing with Castro.

Carter entered office with the announced intention of trying to bring about a U.S.-Cuban rapproachment. As a result, he has never had a wide following in the Cuban community, even though his recent relations with Cuba have been stormy, primarily because of Cuban military involvement in Africa.

In any case, the sources implied, the administration apparently feels that any political losses caused by the decision to take a hard line against massive new Cuban immigration are preferable to the problems and tensions that might be generated elsewhere in the voting population if entry in large numbers were allowed.