The United States, responding to "crisis circumstances in Southeast Asia," has decided to double the number of Indochinese refugees allowed to enter this country in the coming months. Attorney General Griffin B. Bell announced yesterday.

Before cameras and reporters in an unusual public consultation with the House Judiciary Committee, Bell said he is planning to admit 21,875 more Indochinese, about three-fourths of them Vietnamese "boat people" and the rest Cambodians, between now and May 1. This will be in addition to the existing allowance of 25,000 Indochinese refugees per year.

Bell also reported that the United States is prepared to screen and admit about 3,500 political prisoners released from Cuba, in keeping with the announced intention of President Fidel Castro to let them go. Bell said U.S. officials would try to screen and admit about 400 per month after case-by-case study.

In addition, the attorney general said the government has decided to admit 1,000 more Lebanese victims of civil strife in that country who would not otherwise be eligible to come here.

Bell said he is "not comfortable" about alleviating such international problems involving large numbers of people by using his "parole authority" to waive immigration restrictions in special cases. At one point he said he doubts his own power to take such extensive actions.

While calling for new legislation to create broader powers and policies on admission of refugees, Bell said he will go forward with the actions announced yesterday unless appropriate congressional committees instruct him not to do so.

Judiciary Committee Chairman Peter W. Rodino Jr. (D.-N.J.), pointing out that his committee had never objected to the use of parole authority in the past, said, "the facts are very compelling" in the cases Bell presented. "The question is, where do we stop, how far do we go" Rodino added.

As described by Bell and State Department official, the recent surge of "boat people" from Vietnam has created an unexpected world problem of serious proportions, which no end in sight. At the present rate of exodus the United States projects a buildup of about 120,000 boat refugees in Southeast Asia by next spring, more than twice the number now in temporary camps, despite the actions announced yesterday.

Bell agreed with a lawmaker's observation that those fleeing Vietnam no such boats as the much-publicized Hai Hong present new situations because they are largely middle-class people threatened with severe adjustments in their way of life, and because of reports that many of them paid large bribes, perhaps with government connivance to buy their way out.

According to letters to Bell from Secretary of State Cyrus R. Vance, recent U.S. actions regarding "boat people" were finely calibrated for effect on other nations. Vance asked Bell on Nov. 15 to approve entry of 2,500 new boat refugees, the number aboard the stranded freighter Hai Hong, in order to convince Malaysia to grant temporary heaven to those people.

At the time, the U.S. action was limited to this relatively small number "to avoid telegraphing to other governments that we might assume more of the responsibility" for tens of thousands of ther escapees coming out, Vance said.

On Nov. 21 Vance asked by letter for the other Indochinese actions taken yesterday so that at the coming Geneva meeting with other nations the United States will be "in a position to demonstrate that we are also willing to do more."

More than 2,000 Indochinese refugees now in Malaysia will be flown to the United States within the next three weeks to demonstrate the American commitment to the boat people, officials said. Most will be from camps in Malaysia but some with special U.S. ties may be from the Hai Hong.

In discussing the Cuban problem, Bell said the U.S. government had been "very disappointed" to find that lists so far provided by Havana contained mostly names of political prisoners previously released rather than those currently behind bars. The attorney general said he will give priority to current prisoners and those released from prison since Aug. 1 because "if you're in jail you're in worse shape than if you're out walking around."