Style has often overwhelmed substance in the regime of Ronald Reagan, and never more than in his first official appearance 10 days after cancer surgery.

As usual, Lady Luck was in attendance. After a siege of stifling mugginess, she gave him a superlative summer day Tuesday.

And then, to avoid any odious comparisons with his pre-operation self, she gave him as a guest a man, Li Xiannian, only two years older but almost doddering. Only Reagan, who didn't look so marvelous, would, at such a delicate moment, draw a visitor whom he could decently help down the steps of a reviewing stand.

Reagan, as a matter of fact, looked and moved like someone who had recently had a major operation. His jaw line appeared mottled and lumpy, and his usually rosy complexion had a hospital pallor.

But there he was, cheerfully going about his chores, stirring sympathy and admiration. Who on the right wing is going to complain that the president has gotten out of bed to greet a communist, the leader of a nation that he has spent much of his adult life denouncing? No, criticism dies in the face of Reagan's gallant resumption of the daily grind. Who but the Gipper could encounter the killer disease and come away smiling?

The left was not happy that, later in the day, a pact for Chinese purchase of nuclear reactors and nuclear technology was signed without the usual written guarantees that the buyer would not use the technology to make weapons or share it with a third party.

An "oral agreement" with the Chinese has been reached. It seems at best slovenly, at worst a threat to the Nuclear Nonproliferation Act of 1978, which Reagan must enforce, although as a presidential candidate he said that proliferation was none of our business.

But a handshake deal with a communist? What must Mikhail Gorbachev think? He certainly doesn't expect any winks from Reagan on an arms control agreement.

Sen. John Glenn (D-Ohio) was the only one who raised questions, pointing out that ambiguities and gaps in a nuclear agreement with India led to its explosion of a nuclear bomb. "I agree that selling the technology to China improves our relations with them," Glenn said, "but if everything isn't absolutely clear, it could poison that relationship. I want to study the fine print."

But in the drama of the fabulous recovery, the startling "gentlemen's agreement" with a communist receded.

Reagan was always more concerned about the United States as a "reliable supplier" of nuclear technology than the specter of nuclear materials' falling into the wrong hands. He can say it is a matter of business and make the arguments that often accompany huge weapons sales: if we don't sell them, someone else, by whom we generally mean the French, will. Three billion dollars is involved.

In the afternoon, the president received a small setback, unexpected in view of the proliferating deference. He has long craved a line-item budget veto, which would allow him to blue-pencil uncongenial allocations by Congress. But he lost a vote to end a filibuster by those unwilling to cede to him, in writing, their last shred of power even when he is sick.

Naturally, no one spoke of his proclivity to delete school milk money in favor of missiles, or his preference for military installations over shelters for the homeless. No one wishes to speak ill of him when he is soldiering on, giving hope and encouragement to those who turn their faces to the wall when told they have cancer.

But the Senate, which is usually his footstool, managed to remember that he is, after all, mortal, and that if he might be trusted with the new weapon, others might not be worthy of such faith. A reference by Democratic Leader Robert C. Byrd (W.Va.), one of the filibusterers, helped concentrate members' minds.

How, Byrd asked, would the Republicans feel about having a Lyndon Johnson at large with a blue pencil in his holster? And would Democrats sleep soundly if a Richard Nixon were so armed? Senators shuddered, risked being thought callous toward the convalescent, and voted no.

But the defeat was heavily cushioned. Instead of talking about Reagan's setback, people babbled in bafflement about Teddy Kennedy's turnabout. The Senate's foremost liberal came around to Reagan's view. Was it because he wished his brother John had had the power as president? Or because he is headed for the Oval Office himself and wants what he called a "tool to eliminate military waste"?

Or was it a sympathy vote? That's the new Washington calculation.