The Consumer Product Safety Commission is expected to reverse its policy next month and try to prevent the export of garments treated with Tris,a cancer-causing agent used as a fire-retardant, according to sources in the agency.

Last October the commission voted to permit the export of millions of Tris-treated garments, despite the fact that it ruled in April 1976 that such clothing could not be sold in the United States.

At that time, the commission said it had interpreted the Federal Hazardous Substances Act as saying that the CPSC "lacked authority to seize or otherwise interfere with any Tris product that is labeled and marked for export."

Under its own mandate, the commission is allowed to seize any Tris-treated garments destined for domestic sale.

But, with the appointment of two new Democratic commissioners in the past two months and the announced departure of Republican Chairman S. John Byington, the political stance of the commission has shifted and there is growing support to take actions consistent with the administration's human rights stance round the world.

The third Democrat on the five-member commission is David Pittle, who is the heir apparent to the chairman's position. Last November he dissented from the commission's decision not to fight exports.

At that time he was concerned that the CPSE ruling would allow a U.S. clothing manufacturer to take Tris-treated clothing off the domestic market, relabel it for foreign sale, and ship it overseas.

CPSE staffers now say that it appears they have the authority to stop overseas sales of garments originaly made for domestic sale, although they are not sure about garments made for export. Most of the garments in question, however, were first produced for sale in the United States.

Pittle now has the apparent support of his two new Democratic colleagues, Susan King, and Edith Barksdale Sloan. As the top consumer affairs officer of the District of Columbia before joining the commission, Sloan was an outspoken critic of Tris and the sale of Tris-treated garments after the ban had been imposed.

In an internal memorandum sent to fellow commissioners and the CPSC general counsel last week, Sloan said the present export policy "is a critical one which requires an immediate decision."

"Reconsideration of this issue by the commission is inescapable," she said. "The social, political, economic and ethical implications of exporting products already banned by the commission to primarily Third World or developing nonwhite countries demands, in my opinion, distinct and timely consideration by the commission."

She said the commission should deal with Tris immediately, and then move on to other products.

In her confirmation hearings, King said she felt the CPSC had authority to ban exports. "If a court should rule otherwise," she said, "I hope that the Congress would remedy the situation by legislation."

Rep. Henry Waxman (D-Calif.) is said by CPSC staffers to be preparing such legislation. Last year, Byington reported to Secretary of Commerce Junita M. Kreps. U.N. Ambassador Andrew Young and the World Health Organization that he felt that legislation would be needed for the CPSC to enforce anti-exporting sanctions.

Next Friday, CPSC attorneys are going to brief the commission on its legal options, and the commission is expected to vote its anti-export policy on May 11.