The Reagan administration might be willing to support protectionist legislation pending in Congress if changes are made to make the bills more acceptable, Clayton Yeutter, the new U.S trade representative, said yesterday.

Yeutter also said in an interview that he hopes Japan's new plan to reduce nontariff barriers to imports, expected to be announced today, will be substantial compared with its last attempt at opening its markets, which he described as "not terribly significant."

Although Yeutter said that the administration might support modified versions of some protectionist bills, he did not say what those modifications would have to be.

"I don't want to prejudge any legislative proposal, because obviously those can be amended and altered as they move through the legislative process, and it's conceivable that some of those pieces of legislation could be changed in such a way to make them acceptable in trade-policy terms or acceptable in terms of tbe principles that the administration believes," Yeutter said.

The administration has maintained that it favors free trade and opposes protectionist legislation. However, it has come under increasing attack by Congress and lobby groups for the sharply rising merchandise trade deficit, which is blamed for the loss of factory jobs and weakness in the agricultural and manufacturing sectors.

Numerous bills have been proposed to place quotas or tariffs on certain imports. One popular bill would place a 20 percent tariff on all products imported from Japan, and another would place quotas on textile imports.

Yeutter said that the administration's first choice for combating record trade deficits and the increasing market share of imports from Japan and other countries would be negotiations.

If the administration fails in its negotiations with Japan and other countries to open their markets to U.S. goods, Congress has every right to pass legislation redressing those wrongs, Yeutter said.

But Yeutter acknowledged that he has little time to head off congressional efforts to keep out imports. "Many of these issues will come to a head this fall" in Congress, he said.

Members of the Senate Finance Committee sharply criticized the administration during Yeutter's confirmation hearing last month for failing to stem the rise in the merchandise trade deficit. The deficit, a record $123 billion last year, is expected to reach between $140 billion and $150 billion in 1985.

In addition, economic growth has been sluggish over the last four quarters, largely because the influx of imports has siphoned off business from domestic producers.

Yeutter has said that he blames the trade deficit on the high value of the dollar, which makes exports expensive relative to imports. However, he said he opposes -- as does the administration -- intervening in foreign exchange markets to drive the dollar's value down.

Yeutter said that the best way to get the dollar down is through significant reduction of the federal budget deficit, which would lead to lower interest rates and make U.S. assets less attractive. In turn, foreign capital flows to the United States would be slowed, and so would the demand for the dollar, driving down the currency's value.

However, Yeutter also acknowledged that interest rates have declined for about seven months with little, if any, effect on the dollar.

Yeutter also said the United States and the European Community have agreed to suspend for one week retaliatory measures against each other that were to have begun in the last several days.

Yeutter said the United States, pending negotiations with the EC, will suspend plans to implement increased tariffs on EC pasta, and the EC agreed to put off increased duties on U.S. lemons and walnuts. Yeutter said he hopes to make some progress in the next few days.