Crown Prince Fahd, the Saudi administrative ruler, today praised the Reagan administration's efforts to win approval for the planned sale of radar planes to Saudi Arabia and declared that U.S. senators should realize they would best serve their country's interests by going along with the deal.

Responding to questions at a news conference, Fahd seemed to go out of his way to commend the administration's campaign to persuade Congress not to block the $8.5 billion sale of the Airborne Warning and Control System (AWACS) aircraft and other weaponry, and he drew a clear distinction between the White House and the sale's opponents in Congress.

But at the same time, Fahd's comments seemed to parallel warnings from lower ranking members of the royal family that the broad tissue of commercial, military and strategic relations between Riyadh and Washington could be harmed if the proposed sale falls through.

"The president and members of his government are making all possible efforts to make sure the kingdom gets the AWACS," Fahd said. "We know the Congress sometimes decides contrary to the president. But we have every confidence in the president, who is doing his utmost."

Fahd spoke at the conclusion of a two-day visit here by President Francois Mitterrand of France and as the Reagan administration seeks to work out with the Saudi government a compromise arrangement that will have a better chance of congressional approval, possibly one involving U.S. controls over use of the planes and security surrounding their secret components. Pending the outcome of these efforts, Fahd said he would not reveal specific Saudi positions in the contacts or predict what Saudi Arabia will do if the Senate vetoes the sale.

"We will wait for the facts to take the adequate response," he said. "So far, this has not happened . . . . Members of Congress should realize that it is in the interest of their country to approve the sale."

A high-ranking Saudi official and member of the ruling family said yesterday that the kingdom will refuse any agreement that includes U.S. command or obligatory presence of U.S. crewmen beyond the training period. But it is not known whether Fahd, who along with his brother, King Khalid, has the final decision, intends to remain firm in that stance as the White House tries to fashion a deal it can sell to Congress.

Fahd's care to distinguish between President Reagan's White House and Senate opponents of the sale appeared designed to avoid turning the AWACS decision into an "either-or" turning point despite its symbolic and practical importance to the kingdom.

The AWACS planes could, in Saudi eyes, be replaced by British-made Nimrod reconnaissance aircraft. According to the high Saudi official, the Nimrod has been tested here and Britain could begin delivery in 1985.

British Prime Minister Margaret Thatcher reaffirmed during a visit to neighboring Bahrain over the weekend British willingness to sell Nimrods to Saudi Arabia if the AWACS deal collapses.

But diplomats here say, nonetheless, that a blocked AWACS sale would cast a pall here over the idea of doing business with the United States in general. French officials accompanying Mitterrand, for example, made no secret of their hopes to increase French sales to the kingdom to reduce a trade deficit that reached about $3 billion in the first six months of this year, chiefly due to French imports of Saudi oil.

Despite initial Saudi fears of Mitterrand's socialism and the Communists in his government, Paris appears to have convinced the royal family that France can remain a good friend and business partner.

Mitterrand came to power May 10 with a reputation as a warm friend of Israel. Although he has continued proclaiming that friendship, he also has reassured the Saudis with expressions of policy on the Arab-Israeli dispute differing little from that of his predecessors, including a call for Palestinian rights and "state structures they choose for themselves."

Mitterrand also stressed -- before leaving Paris and in his talks here with Khalid and Fahd -- his general endorsement of Fahd's recent eight-point Middle East peace plan as a starting point for an effort to broaden the flagging Palestinian autonomy talks. He did so again today in his joint news conference with Fahd, calling it "certainly a positive way to start the search for peace."

Saudi rulers have welcomed French support for Fahd's plan as a counterbalance to its failure to stir enthusiasm in Washington. Although at his meeting with Fahd in Spain two weeks ago Secretary of State Alexander M. Haig Jr. promised to study the plan, Saudi sources said, the crown prince is disappointed at the Reagan administration's apparent lack of interest.

Fahd, discussing his plan today, emphasized on several occasions that it was put forward as a "basis for discussions," not as a take-it-or-leave-it proposition.

"These principles do not exclude debate," he said, apparently referring to Arab countries that may want to harden the terms as well as to supporters of Israel that may want to soften them.

A Saudi official with access to Fahd's thinking said yesterday that the eight points, including a demand for return of East Jerusalem to Arab control, were not negotiable. Fahd's declaration a day later that they are indeed negotiable underlined the differing and sometimes contradictory currents in the royal family.

Fahd generally is considered the strongest advocate of close relations with the United States and patience in efforts to get around U.S. support of Israel, which most Saudis see as an obstacle to peace. It was seen as significant that his plan included, for the first time in such explicit, public language, a Saudi commitment of respect for the right of all states in the Middle East to exist in peace.

Mitterrand singled out this aspect, saying: "This is progress."

He avoided mention of the demand for a return of Arab East Jerusalem, which runs head-on into Israeli insistence that the entire city is Israeli and must remain so.