WHEN Ronald Reagan meets Deng Xiaoping in Peking today, the pageantry will be great theater to watch as it is telecast throughout the world. The symbolism of the de facto leader of the Orient's giant welcoming the West's most influential elected leader - only the third such meeting in a dozen years - is obviously of some moment.
Only a Moscow trip for Reagan would have outranked the Peking visit in significance for the American President domestically, and in implications for global affairs.
For Mr. Reagan and American conservatives long antagonistic to Communist China, his mission represents a muting of ideological objections and a step toward more realistic commercial relations with the People's Republic.
But beyond this, Washington should be modest in its assessment of Mr. Reagan's mission. China, with thousands of years of history behind it and a quarter of the earth's population today, can afford to be patient toward the American President. Mr. Deng can play Washington's emissaries off against Tokyo's and Moscow's.
Playing the so-called ''China card,'' edging closer to Peking strategically to counter Soviet ambitions in Asia, is a limited hand for Washington. True, the Soviets have helped prepare a welcome for Reagan in Peking with their latest bombing of the Panjshir Valley in Afghanistan. But they have observed the American leader's problems in Lebanon and Central America, the turmoil following or accompanying the American projection of force. The Chinese are not all that interested in military alliances.
The Chinese under Deng have two overriding interests - Taiwan and modernization. On Taiwan, they will press Reagan to move more decisively to reduce military aid to the island, to end the ''semiofficial'' contacts with Taiwan, and to spell out his plans for Taiwan in a second term.
Although the Chinese have signaled they do not intend to embarrass their visitors, they will push in private on all three fronts - dollar levels, contacts under the Taiwan Relations Act, and future Reagan policies.
If Peking's message is that Taiwan is as important to Deng as modernization, Reagan's ploy is to disentangle the two. Hence Washington's emphasis on sales of nuclear facilities to Peking.
In the news media, much is made of the ''new China'' under Deng: capitalist-style incentives for farmers, Western makeup, private cars. Deng will be discreet. But he is not expected to go along with letting Reagan ''buy'' Taiwan off the table with a pitch for the China trade.
Nor does the Reagan visit say the last word on the President's own views of China - or the two Chinas. His administration has barely trimmed arms sales to Taiwan, from $697 million in fiscal 1983 to $680 million this fiscal year, under its August 1982 communique with Peking promising to ''reduce gradually'' arms sales to Taiwan. At that rate, ending the Taiwan arms connection would take 40 years - a very slow boat indeed, even by Chinese standards.