A U.S. congressional delegation ending a four-day visit here said today that the United States must do more to stem the flow of Indochinese refugees, many of whom they say are "economic migrants" rather than victims of persecution.

In a news conference, some of the congressmen -- part of a bipartisan group of House Judiciary Committee members that includes several key subcommittee chairmen -- said they would recommend changes in U.S. refugee laws and predicted closer congressional scrutiny of administration proposals on the admission of refugees during consultations due to begin soon.

The Reagan administration unveiled its overall plan to limit immigration during joint hearings late last month of the House and Senate subcommittees on immigration.

The visit, during which nine congressmen toured refugee camps in Thailand and conferred with Thai officials, appeared likely to renew a debate in the United States on the definition of a refugee. Such a debate resulted in a bureaucratic dispute this spring between the State Department and the Immigration and Naturalization Service that delayed resettlement of hundreds of Asian refugees.

It also raised concern among refugee officials here that anticipation of a stricter U.S. policy could cause harsher treatment of arriving refugees by Thailand and other Southeast Asian countries. One dismayed official noted that some countries have responded to a buildup of Vietnamese refugees in the past by pushing the boats of new arrivals back out to sea.

Thai government officials already have perceived a changing American mood that they fear will lead to restrictions on resettlement in the United States and saddle Thailand with an increasing number of refugees.

In the past few months, therefore, Thailand has begun to take a tougher line to discourage Indochinese from leaving their homelands. Some refugee camps have been closed to new arrivals, and more than 400 Vietnamese who fled their country overland have been refused permission to leave the volatile Thai-Cambodian border area for safer camps.

In addition, Thai officials have warned that after Aug. 15, arriving Vietnamese boat people would be ineligible for resettlement abroad and would be put in detention camps until they were ready to return home.

The visiting congressmen could offer no alternative to resettlement for Vietnamese who flee their country, but they stressed that a way must be found to discourage them from leaving in the first place through what one delegate called "humane deterrence." Two of the congressmen criticized U.S. refugee quotas, the Voice of America and the U.S. 7th Fleet, accusing them of attracting refugees.

The chairman of the delegation, Rep. George E. Danielson (D-Calif.), said existing U.S. law "defines a refugee in an inadequate manner, and we have to straighten this out." He said his constituents were "concerned, even alarmed" about the influx of refugees and that something must be done to "cut off the flow."

Danielson, chairman of a subcommittee on administrative law and governmental relations, predicted that "the refugee law will be worked over" and that the government's program "will be reviewed." He added that he would recommend the removal of refugee quotas entirely because they "constitute a pull factor, a magnet for refugees."

The quota for Indochinese refugees currently provides for admission of 168,000 people during fiscal 1981, which ends in September.

Rep. Romano L. Mazzoli (D-Ky.), chairman of the key subcommittee on immigration, refugees and international law, said congressmen would no longer simply accept the administration's figures on the number of refugees to be admitted. He said consultations with the administration on the matter would be much more detailed this year.

Mazzoli would not predict how much refugee admissions might be cut, but said that any reductions could be gradual. He acknowledged that such measures might lead to "fundamental changes" in the policies of Southeast Asian countries. However, he said he hoped these would not be drastic.

Reflecting the tougher mood, Rep. Sam B. Hall (D-Tex.) said that during the delegation's fact-finding tour of refugee camps, "we found evidence of people who do not fit the political refugee definition."

Hall added, "There must be a day of reckoning. The United States can't serve as a depository for every person in the world who wishes to leave his country. Word must be sent back that the United States is getting ready to draw the line."

He said he thought the Voice of America has "contributed to the problem" by describing the United States as "a land of milk and honey" and thereby attracting refugees.

The presence of the U.S. 7th Fleet in Pacific waters, where it often picks up Vietnamese boat people, also was mentioned in this regard.

Hanoi has accused the United States repeatedly of encouraging Vietnamese to flee. The Hanoi government even went so far as to charge recently that U.S. tactics included smuggling into Vietnam pills to prevent travel sickness, a charge denied as "absurd" by the U.S. Embassy here.