In what appears to be a major step toward a liberalized visa policy for Communists, the Carter Adminstration has decided to let the Italian Communist Party's daily paper, Unita, open a bureau in the United States.

The news, which was termed "highly significant" by Unita editor, Alfredo Reichin, was relayed to her paper yesterday by the American embassy here. An embassy spokesman told Unita representatives that a visa will be granted to Alberto Jacoviello, a controversial Communist journalist who has long been slated for the U.S. post.

Acceptance of the request of the Italian Communist daily, which has a circulation of 350,000 is a major departure from previous practice. While newspapers from Communist-ruled Soviet bloc countries have long had bureaus in the United States, the opening of a permanent bureau by a Communist news organ outside the Soviet bloc will give a Euro-Communist Party paper its first opportunity to report directly from the United States.

A political officer at the American embassy said today, however, that the decision regarding Jacoviello's visa was not to be considered a gesture toward the Italian Communist Party itself.

"It is just one element of an overall policy designed to promote cultural exchange and freedom of information in accordance with the Helsinki accords," he said, adding that it would be regrettable if the Italian press were to give the event a broader political interpretation.

He said that there has been no change in the American policy of non-interference but not indifference to Italy's Euro-Communist parties, with emphasis on U.S. preference for parties and governments of a "democratic" nature.

There have been growing complaints from anti-Communists political and business leaders that the Carter administration is permitting the impression to be created that whether Communist parties come to power in Italy and France is a matter of indifference to Washington. Conversely. European leftists have been using the apparent ambiguity in U.S.policy to suggest to middle-class voters that is is safe to vote for the left.

Unita's visa application has been pending since the Nixon administration.

In the past the leftist press in Italy has tended to highlight visa decisions regarding Communists as evidence of changes in U.S. policy that could enhance the status of powerful Communists.

But editor Reichin said today that Unita does not intend to capitalize politically on what he termed "a long awaited decision."

"We are simply extremely pleased," he said, "and we know this will lead to more thorough mutual understanding and greater knowledge on our part about the United States."

The decision to grant a visa to Jacoviello, a two-time foreign editor of Unita who will be visiting the United States next week for the third time, suggests a climate in contrast with that of two years ago, when two ranking Communists were refused entry permits.

A 1952 law makes a present or past member of a Communist Party ineligible for an American Visa, but in recent years the State Department had managed to persuade the attorney general's office to grant waivers to Italian Communists seeking to visit the United States "for legitimate reasons."

Since the start of the Carter administration, however, the number of highly placed Communists visiting the U.S. appears to have increased. This year at least two well-known Communists have received visas: an economics professor from Venice, three members of the Central Committee and one member of the smaller directorate or Politburo.

An embassy spokesman said that the current trend is "part of Carter's world-wide philosophy" and that a general study regarding possible ways of revising or circumventing the 1952 McCarran Walter Immigration Act is in progress in Washington.

Nevertheless the choice of Jacoviello, who is thought to be planning to open a bureau in New York or Washington in September, is significant. Jacoviello, who was on vacation and unavailable for comment, is reported to believe that Communists anti-Americanism of the Cold War period has not always allowed Unita to speak of the United States objectively.

A noted political commentator who holds no formal position in the party hierarchy, Jacoviello has twice quit as Unita's foreign editor because of disagreements with party policy.

In 1956 he resigned in anger over the paper's support for the Soviet invasion of Hungary, and later he was forced to resign because of a difference of views over China. Last year Jacoviello was again briefly at odds with the party leadership when in an article that appeared in the French daily Le Monde he criticized the Italian Communists for delay in adopting a China policy independent of that of the Soviet Union.