The Palestine Liberation Organization's governing body, under last-minute pressure from its radical wing, today endorsed a resolution saying that it "rejects considering" President Reagan's Middle East peace plan as a sound basis for solving the Arab-Israeli conflict.

At the same time, a senior aide to PLO Chairman Yasser Arafat suggested that the Reagan plan could be made acceptable with just one change to endorse "self-determination" for the Palestinian people. Clearly signaling a willingness to make concessions--even excluding the PLO from talks on achieving Palestinian self-government in territories now occupied by Israel--Salah Khalaf told U.S. reporters that he and Arafat were willing to risk splitting the PLO if the United States replied positively.

At the final session of its nine-day meeting here, the Palestine National Council approved using the Arabic word "rafd," or "reject," to describe its position regarding the Reagan plan. Arafat strongly opposed use of the word.

Arafat supporters said that the language stopped short of an outright rebuff to the U.S. initiative, leaving the chairman with sufficient room to maneuver in still-problematic talks about the future of the occupied West Bank and Gaza Strip.

After six hours of wrangling, PLO radicals succeeded in marginally toughening the language of a resolution recommended by the political committee yesterday. The panel had said that the Reagan plan was "not accepted" as a basis for a solution.

The official English translation watered down the key passage of the resolution approved today by avoiding the word "reject," saying ". . .the Palestine National Council refuses to consider it the Reagan plan as a sound basis for a just and lasting solution to the Palestinian problem and the Arab-Israeli conflict."

Ahmad Abdel Rahman, the council's official spokesman and an Arafat loyalist, insisted that there had been "no basic change" from the political committee's recommendation.

The Reagan plan unveiled Sept. 1 called for establishment of a Palestinian entity in the occupied territories to be associated with neighboring Jordan. The council here called for creation of an independent Palestinian state, rejected by Reagan, that then would join Jordan in a confederation.

The council, an unofficial parliament-in-exile, also reelected Arafat as chairman and endorsed the Arab League's Fez plan for peace. The plan implicitly offers to recognize Israel in return for establishing a Palestinian state.

As if to soften the impact of the counterattack against the moderates led by the chairman, Arafat aide Khalaf said in an interview at his seaside villa near the Club des Pins conference hall, "If the Reagan plan is improved, for example, by adding just one word . . . things could change completely. The word is self-determination."

"If the U.S. government was in good faith toward the Palestinian people--and I do not say toward the PLO, just the Palestinian people--and accepting the fact that in the eyes of the U.S. government we are the bad Palestinians," Khalaf said, then "let your government recognize the right of self-determination not to us, but to the Palestinian people, including the right to the creation of a state and excluding the PLO representatives."

The issue of the PLO's role in talks is important because the United States opposes allowing PLO involvement until it explicitly recognizes Israel's right to exist.

"I can assure you that Arafat and myself would agree to being excluded," Khalaf, better known as Abu Iyad, said. He said that he was speaking because "I want to be convinced that the U.S. government is not hostile to the Palestinian people itself."

He said the Reagan administration "should offer self-determination directly to the Palestinian people, not through King Hussein of Jordan" but through the mayors of the occupied territories, "since the U.S. government considers the mayors to be the legitimate representatives."

Such an approach would provide support for Arafat in his rivalry with PLO radicals, who have opposed his preference for having non-PLO mayors represent the Palestinians in a joint delegation with Jordan that would negotiate with Israel under the Reagan plan.

Left unmentioned was the assumption that the PLO could influence and even might control any vote on the future of the West Bank and Gaza Strip.

Arafat managed to remove potentially damaging language from a resolution on relations with Jordan. The deleted passage rejected any "imperialist-Zionist schemes" designed to undercut the PLO's authority as the sole, legitimate representative of the Palestinian people by "mandating, authorizing or sharing" that role with "other parties."

Exactly the same wording was used in a general preamble that did not refer specifically to Jordan.

Arafat also succeeded in putting potentially explosive future relations with Egypt, and with Israelis in favor of an independent Palestinian state, under the responsibility of the executive committee, which he in effect controls .

Proof of his renewed power over the executive committee was his success in obtaining reelection of all 14 other members without opposition. He was unable, however, to have his new mandate as chairman of the executive committee approved by acclamation by the council, largely because of opposition from the ranks of his own Fatah movement as well as from hard-line radical critics.

Arafat had gone through the motions of asking for approval by acclamation, but some insiders said he had done so solely as a bargaining counter to extract concessions on the wording of key resolutions.

In a closed session Sunday night, for example, he warned critics that unless they gave him his way he might quit, or, as he was reported to have put it, "you try to find another chairman."

His pleas for "flexibility" required for "practical purposes" and a free hand for political maneuvering were heard but not fully approved. Since the PLO's forced departure from Beirut, critics claim that Arafat has invoked the PLO's dispersion across the Arab world to justify ever more free-wheeling diplomacy and to ignore his colleagues.