President Reagan headed west today, his spirits buoyed by a string of domestic and foreign policy successes and the prospects of a two-week vacation at his California ranch.

"The president leaves for a well-deserved rest flying high," said White House deputy press secretary Larry Speakes.

Reagan's happy mood was broken only by attendance this afternoon at a 20-minute memorial service for Nancy Reagan's stepfather, Dr. Loyal Davis, who died Thursday at the age of 86. He had been seriously ill for several months.

Talking with reporters aboard Air Force One en route here, Speakes said that Reagan had told him that the week was "probably the most rewarding or fulfilling week we've had."

He gave four reasons for this feeling: settlement of the China-Taiwan issue, favorable economic news as reflected by the surge in the stock market, congressional approval of a $98.3 billion tax bill, and the agreement on removing the Palestine Liberation Organization from Beirut.

Though Reagan was clearly pleased with Thursday's passage of the tax bill, which was pushed through the House by a bipartisan leadership coalition, he and other administration officials remain sensitive to the views of the 89 House Republicans who opposed the measure.

Speakes said that the president would talk by telephone with Rep. Jack Kemp (N.Y.), leader of the Republicans who opposed the measure, in an effort to keep the breach from widening.

Reagan also plans to sign the tax bill in the seclusion of his mountaintop ranch northwest of Santa Barbara without the fanfare and photographs that have attended the signings of major legislation in the past.

But while his public celebration of his tax-bill victory was deliberately muted, Reagan made no attempt to conceal his pleasure at the week's economic news, especially the lowering of the prime interest rate by major banks.

In the administration, the interest rate decreases and the stock market gains are being hailed as signs that Reagan's economic recovery plan is finally working after earlier false starts.

The foreign policy successes, particularly in the Middle East, were also occasions for White House rejoicing, although Speakes added the cautionary note that the sending of 800 Marines to Lebanon as part of a multinational peace-keeping force "was not a decision taken lightly."

After the memorial service, which was closed to the media and other outsiders, Reagan spent the night at the residence of Nancy Reagan's mother, Edith Davis, who is incapacitated and confined to her home. On Saturday the Reagans are to fly to Point Mugu Naval Air Station on the California coast and go by helicopter to their ranch.

With Congress in recess, the White House is making no attempt to present the trip as anything other than a vacation. Speakes said the only immediate items on Reagan's working agenda are continued monitoring of the situation in Beirut and a decision on the supplemental appropriations bill.

White House advisers are expected to recommend a veto because the legislation is nearly $1 billion over a previously agreed spending limit, Speakes said. On Monday, Reagan is to fly to Los Angeles, where he is to be the featured speaker at a fund-raising dinner for Pete Wilson, the Republican nominee for the U.S. Senate against Democratic Gov. Edmund G. (Jerry) Brown Jr.