Yasser Arafat's observer and shadow emissary on the sidelines of the Arab League's discussions with the Reagan administration has said he would welcome a U.S. initiative asking for "mutual, simultaneous recognition" by Israel and the Palestine Liberation Organization, but only after there is an agreement to establish a Palestinian state in the occupied West Bank and Gaza.

Khalid Hassan, a key member of the PLO inner circle, said in an interview here this weekend that he would find the U.S. initiative "wonderful," but insisted on the precondition. The PLO demand for an independent state has been one of several obstacles to launching peace negotiations.

The Israelis adamantly oppose an independent state, and the Begin government has indicated a long-range goal of incorporating the territories into Israel. President Reagan's Middle East peace initiative calls for expanded negotiations to reach an interim autonomy accord for the 1.3 million Palestinian inhabitants of the occupied territories.

Hassan signaled a readiness to recognize Israel explicitly, which was more clear than previous, ambiguous statements by Arafat. The PLO leader has indicated publicly a willingness to negotiate for a Palestinian homeland with "all the democratic Jews," but he has refused to clarify what he meant by "democratic."

Hassan said that the PLO would agree to negotiate some eventual "confederation" with Jordan within the vague framework Reagan sketched broadly in his Sept. 1 speech outlining the new American diplomatic effort, but that the Palestinians first must be allowed to establish a separate, independent national identity.

"We first want to have our flag, our passport, army, our policemen, our parliament," Hassan said in a 1 1/2-hour interview in his hotel suite late on Friday. "That means to have our state, and then immediately after that, we will make a confederation with Jordan."

A key moderate adviser in the circle of leadership around Arafat and a businessman and former government official in Kuwait who imports household appliances, the effusive Hassan, 44, flew to Washington from Morocco as the PLO's representative in the Arab League delegation that met with President Reagan at the White House Oct. 22.

But Hassan was barred from attending any of the meetings or from having informal contacts with Reagan administration officials because of a 1975 U.S. understanding with Israel that the United States would adhere to a policy of not recognizing or negotiating with the PLO as long as it does not explicitly recognize Israel's right to exist.

Hassan said the Palestine National Council, the PLO's legislative arm, would meet before the end of the year to consider its formal response to the Reagan peace plan. But he said no date or location has been set for that meeting, where objections are expected to be raised by radical factions of the PLO. Hassan is a member of the council.

Hassan pleaded with Americans to judge the PLO by the "mainstream" of the movement. The hijacking, terrorist elements should no more be viewed as representing the mainstream of the Palestinian movement than underworld drug chieftains and those who have put cyanide in pain-reliever capsules portray the basic American psyche, he said.

Praising "positive points" in the Reagan peace initiative, as Arafat and leaders representing moderate Arab states have previously, Hassan noted pointedly that it was Israel, not the PLO, that rejected outright the U.S. peace initiative. He argued, as have other Arab leaders, that the implicit recognition of Israel in the eight-point declaration in September by six Arab foreign ministers in Fez, Morocco, should be sufficient.

But the Reagan administration has pressured the Arab League delegation to "come out of the closet" and make clear that they are willing to recognize Israel. Privately, the White House and Middle East specialists in Washington have been telling moderate Arab states that they must explicitly acknowledge Israel's right to exist if they hope to influence wary, but volatile, public opinion to a more sympathetic view of the Arab position.

Americans must realize "the unbelievable concessions that have been made by the Palestinians," Hassan said. "I think we have said everything that is needed. They insist to have it in a special wording. Why? Is it humiliation?"

But, he said, "The Reagan plan would have been wonderful if he asked the PLO and Israel to have a mutual, simultaneous recognition."

"Reagan would have been wonderful if he adopted the human element and is not putting all that pressure on the victim," he said.

Although he has been in the shadows during his two weeks in the United States, Hassan has played a more visible role in the international zone around the U.N. complex in New York. And while he has been barred from direct contacts with U.S. officials, he has not been sitting in his hotel suite waiting for the telephone to ring. He was not at the White House meeting with Reagan nor at the dinner with Secretary of State George P. Shultz and Vice President George Bush, but Hassan said that the delegation frequently consulted him as they met with members of the administration.

"If they have any inquiry concerning the PLO , they have to ask me," he said. Hassan said he also has been going through a crammed schedule of meetings and dinners with journalists, members of Congress, foreign policy advisers in previous administrations and some businessmen.

Hassan held several meetings when he was in Washington for the Arab League's Oct. 22 meetings. Last summer, when he quietly visited the nation's capital during the Israeli bombardment of West Beirut, he met privately with Senate Foreign Relations Committee Chairman Charles H. Percy (R-Ill.), an aide to the senator confirmed. The aide said Percy had contacted a senior State Department official before the meeting and was informed that the official "had no objection to it."

Hassan complained that Washington has not placed the same pressure on the Begin government to recognize the PLO as it has on the Arabs to recognize Israel. He also indicated that he still hopes to achieve Arafat's announced goal of winning over American opinion.

"We are not out of patience," he said. "Our problem taught us how to be patient. We have learned that . . . we have to knock on the door until the door is opened."