The American ''open door'' toward Indochina refugees has begun to close.
Until last month, tens of thousands of Vietnamese boat people who survived the perilous passage across the South China Sea and Laotian refugees who swam the Mekong River were reasonably sure of eventual resettlement in the West.
But under a Reagan administration policy that went into effect May 1, many refugees may face a closed door unless they have direct ties to the United States.
Moreover, if the recent experience of refugees from Kampuchea in interviews with immigration officials in Thailand is repeated, each Indochinese may have to prove that he or she is fleeing persecution.
The new policy is regularly announced over the Voice of America. It is intended to discourage future refugees. But at the same time it is designed to convince governments of the Association of Southeast Asian Nations (ASEAN) that the declining number of refugees seeking ''first asylum'' in ASEAN countries after fleeing Indochina will still eventually be resettled in countries like the United States.
According to an official State Department explanation of the policy, refugees arriving in countries of ''first asylum'' after April 30 will be eligible for US resettlement only if they have close relatives in this country, are former employees of the US government and ''the US-based organizations,'' or were officials of the ''pre-April 1975 governments of the Indochinese countries.''
''The United States cannot offer haven to everybody who may wish to leave their homeland,'' said H. B. Cushing, the director of the State Department Office of International Assistance and Protection. ''It is just economically, politically, and socially impossible.''
Last year, Congress, concerned about high unemployment and the cost of refugee programs, reduced the annual ceiling for Indochinese admissions from 168 ,000 to 100,000. Officials estimate that approximately 80,000 will enter the US this year. Since 1975, more than 600,000 Indochinese have been resettled in this country.
State Department officials said that the new policy aimed at reducing what is commonly called the ''pull factor,'' the incentive to potential refugees offered by their knowledge they will be easily accepted by Western nations. The number of refugee arrivals in ASEAN countries has been falling off during the last year - about 4,000 a month from January to April, compared with about 10,000 monthly in the same period last year. Officials hope that fewer refugees will come if resettlement opportunities are closed off.
To reduce further the refugee burden on the ASEAN countries, the Reagan administration is promoting the Orderly Departure Program, under which Vietnamese eligible for resettlement may legally migrate directly from Vietnam. In April, 427 Vietnamese entered the US under this program, while 6,824 Indochinese immigrated from the countries where they first sought first asylum. There are no orderly departure programs from Laos and Kampuchea.
State Department officials acknowledge that the success of the new policy depends on the continued decline in refugees fleeing from Indochina. For ASEAN governments, additional reductions in US refugee admissions are palatable only if the overall number of refugees drops.
However, there is widespread concern that the balance between arrivals and departures could be upset by the recent actions of Immigration and Naturalization Service (INS) officials interviewing thousands of Kampucheans designated for possible resettlement.
The Kampucheans -- survivors of the killings and starvation that wracked their country during the 1970s -- have been confined in camps along the Thai-Kampuchean border for more than two years. Thailand's government, according to an unclassified April State Department cable, has recently pressured the US to accept those eligible for resettlement.
State Department and congressional officials report that INS officers have denied admission to 60 percent of the Kampucheans interviewed in recent weeks, apparently because they cannot prove they will face severe persecution if they return to Kampuchea. In the past, nearly all Indochinese refugees were assumed to be political refugees. The INS actions essentially require each refugee to prove the likelihood of oppressive treatment at home.
One State Department official said that the INS actions are not policy, but represent ''a graphable trend of serious proportions'' that ''could affect our relations with ASEAN'' if the long-term result is a significant drop in refugee admissions.