President Reagan met with Prince Fahd of Saudi Arabia for 70 minutes here today, reportedly without mentioning the adminstration's effort to persuade the Senate to approve an $8.5 billion arms sale to Saudi Arabia.

White House communications director David Gergen said that the arms sale, which the adminstration has called crucial to U.S.-Saudi relations, did not come up in a meeting that was largely devoted to the situation in Lebanon.

Reagan told Fahd that he is sending special emissary Philip Habib back to the Middle East about the second week of November to make a new effort to improve conditions in Lebanon, Gergen said.

Habib's mission last spring resulted in a fragile cease-fire in the bloody war of attrition between Israel and the Palestine Liberation Organization and was given considerable support by Saudi Arabia. Reagan thanked Fahd, the Saudi deputy prime minister, for his nation's aid to Habib's initial effort, Gergen said.

In addition to Lebanon, the two leaders also discussed general problems in the Middle East and the activities of Libyan leader Muammar Qaddafi, Gergen said. Gergen, who did not attend the meeting, told reporters he could provide no details of the discussion of Qaddafi.

Gergen also said he did not know if the situation in Egypt following the assassination of president Anwar Sadat had been discussed.

Although some politicians saw today's meeting as a final opportunity for Reagan to seek new assurances from Saudi Arabia that might win support for the arms sale before the Senate vote scheduled for Wednesday, Gergen said negotiations with the Saudis ended several weeks ago and Reagan never intended to attempt to extract further concessions from the Saudis while at the 22-nation summit meeting in Cancun on economic development.

Reagan did make one effort to boost chances for the sale of five sophisticated AWACS radar planes, air-to-air missiles and other advanced Air Force equipment. He telephoned Sen. Dale Bumpers (D-Ark.) in an effort to win Bumper's vote. However, Reagan reached Bumpers after the Arkansas senator had make public his decision to oppose the sale, Gergen said.

Gergen said the president still believes the sale will win Senate approval. The president plans to use his personal powers of persuasion in an attempt to save the sale at the 11th hour.

"President Reagan will be going full-bore on AWACS Monday and Tuesday and probably Wednesday," Gergen told reporters. He said the president will invite five or six senators to the White House each day to solicit their votes.

The extreme sensitivity with which the Reagan White House views relations with Saudi Arabia as the AWACS vote approaches was indicated by the unusual efforts taken to prevent reporters from asking questions when they were allowed into Reagan's suite briefly so that photographers could take pictures of Reagan and Fahd together.

The White House press office passed the word that no questions could be asked during this "photo opportunity." Usually, the president responds to two or three questions during such sessions.

Gergen explained that reporters' silence was necessary on this occasion out of respect for the dignity of the president and out of respect for his guest.

When Reagan led Fahd on to the suite's patio and pointed out to his guest the tape on the green Astro-Turf, which marked where they should stand, a reporter asked Reagan about the impact of the Senate vote on U.S.-Saudi relations.

"Let me just say, this is a photo opportunity with our guest and we agreed to take no questions," the president replied.

Secretary of State Alexander M. Haig Jr., Treasury Secretary Donald T. Regan, White House chief of staff James A. Baker III, and national security adviser Richard V. Allen, took part in the luncheon meeting.

A second bilateral question that cut across the U.S. delegation's emphasis on international development issues at this summit was raised in an television interview with Haig this morning.

The secretary was asked whether he got the impression from Chinese officials here that U.S.-Chinese relations would be downgraded if Washington sells advanced weapons to Taiwan. Reagan and Haig met Wednesday with Chinese Premier Zhao Ziyang.

"I think I've had that impression from the outset of our incumbency here in Washington," Haig replied on the "Morning" (CBS,WDVM) news program. "The Chinese have been very clear about expressing concerns. And if we handle the armament question with Taiwan insensitively, its going to result in a setback in our relationship.

"I don't think its shattering because those relationships are built on more profound, fundamental interests. But it would represent a setback. Now we have told them that we are conscious of that, but we have also told them that they must be conscious of our obligations to meet our commitments to the people of Taiwan . . . . But we have made no decision to sell anything, and we are now looking very carefully at what Taiwan's true defense needs are in the aircraft sector.

[Later in the day, Reagan met with Algerian President Chadli Benjedid and personally thanked him for Algeria's role in helping to win the release of 52 American hostages held in Iran for 444 days, United Press International reported from Cancun.]

[Algeria acted as go-between in negotiations between Washington and the Tehran government that ultimately led to the release of the hostages in January. The Reagan administration previously had praised Algeria for its even-handedness in settling the hostage matter.]