The Reagan administration, having informed Saudi Arabia of its willingness to supply a major package of new military gear, is preparing what an official traveling with Secretary of State Alexander M. Haig Jr. described today as the "substantial task" of convincing Congress to approve the sales.

The U.S. decision, conveyed to the Saudis in recent days by a message from Defense Secretary Caspar Weinberger, became known as Haig left the oil-rich kingdom, the final stop of a four-day Middle East tour, and headed for Western Europe.The main items in the package are five airborne warning and control system (AWACS) planes.

An unidentifiable "senior official" who travels with secretaries of state told reporters aboard Haig's plane that "I don't pretend we don't have a substantial task ahead of us" to convince Congress in view of strong opposition from Israel, especially to the AWACS.The official went on to say that the administration will explain to Congress the motivations for its decision, the implications, purpose and capabilities of the equipment and the consequences of rejecting the sales.

"We'll get it through," he declared.

The military sales to Saudi Arabia will thus provide a test of the administration's Middle East policy of arming diverse regional allies and seeking a workable "strategic consensus."

Four of the big airborne reconnaissance planes were dispatched to Saudi Arabia last September, at a high point of tension in the Iranian-Iraqi war. The planes and their U.S. crews and support personnel of about 400 are still in Saudi Arabia and, if the Saudis have their way, will remain until the kingdom's own five AWACS roll off the American assembly lines beginning late in 1985.

In the interim, the Saudis are interested in working out a lease contract for the presently assigned aircraft, in order to contribute a greater share of their upkeep costs and to insure that they will remain at their desert station tense situations that may arise elsewhere. Discussions on such a lease are scheduled to begin soon.

Saudi Arabia considers the supply of the additional military equipment, including fuel pods and air-to-air missiles for F15 warplanes, KC135 flying tankers and the AWACS, a litmus test of its U.S. connection.

Israeli Prime Minister Menachem Begin, in a joint press conference with Haig two days ago, called the much-rumored AWACS sale "a very serious threat to Israel" and reportedly made it clear to Haig in private that his government will wage a full-scale fight against it on Capitol Hill.

The conflict over the AWACS sale is the most tangible expression of the strains among the Middle East enemies who Haig sought to bring together in the past four days in an anti-Soviet "strategic consensus."

Haig is spending tonight in Spain. Summing up his travels as he flew toward a brief Rome stopover to see Italian Foreign Minister Emilio Colombo, the secretary said he found "a unanimity of views" among Middle East leaders about the need for more effective efforts against "direct and indirect Soviet imperialism in the region."

The Saudis, like Haig and most others in the area, privately acknowledge that little can be decided or done about the Arab-Israeli dispute in advance of the Israeli election June 30. This has given the Reagan administration a respite from demands for an immediate start on renewing or amending the Camp David process, which engages Israel and Egypt but continues to be strongly opposed by Jordan and Saudi Arabia.

Haig emphasized at each of his Middle Eastern stops that an anti-Soviet "strategic consensus" and progress in Arab-Israeli peace are not inconsistent aims, but are "mutually reinforcing."

The Saudis and Jordanians maintained that progress in settling the Palestinian issue would ease the most serious internal threats to the region, including some major inroads for the Soviet Union, but they did not see a stronger anti-Soviet policy helping solve the Palestinian problem.

The "senior official" reported that Haig found at every Middle East stop long-standing doubts about America's staying power, its leadership, its willingness to meet its commitments, and above all its willingness to stand up to Soviet aggression.

The official claimed that "the minute we addressed these issues" to show a reassertion of U.S. global responsibilities and consideration of friendly nations' views this "automatically turned each of our hosts into an entirely different mood and attitude." He added, however, that the key test will be the actions taken, "not our professed words or adherence to shifting policy."