Ronald Reagan and George Bush, the Republican presidential and vice presidential nominees, took steps today to assure China that a Reagan administration would make no change in Sino-American relations.

At a news conference here preceding Bush's departure on a nine-day trip to China and Japan, Reagan said that, while he is in favor of upgrading relations with Taiwan, he also intends "to continue working towards increasing this relationship with the People's Republic of China."

Asked whether this means he no longer advocates formal relations with Taiwan, Reagan replied, "Somehow what I have said slid into being taken as diplomatic recognition of Taiwan."

Reagan is a longtime supporter of the nationalist regime on Taiwan. "No more Taiwans" was one of his favorite slogans on the primary campaign trail. And last May, campaigning in the Michigan primary, the former California governor said that a Reagan administration would seek to reestablish "official relations" with Taiwan, which he called "the true Republic of China."

China-watchers believe it was that statement that prompted Peking, through the People's Daily and the New China News Agency, to warn that a Reagan presidency could destroy Sino-American relations.

Reagan "intends to turn back the clock and conduct American foreign policy as if there were two Chinas," admonished the People's Daily. "It will be very dangerous."

So last month the Reagan campaign began to build bridges to the mainland.

First Reagan foreign policy adivser, Richard V. Allen made a statement in which he assured the Chinese that a Reagan administration would not make any change in the present relationship.

And Reagan, at the time he announced Bush's trip, said that while he intended to "maintain" relations with the Republic of China of Taiwan, "this does not mean in any way that we want to or intend to lessen the efforts to build a friendship with the People's Republic of China on the mainland."

Reagan said today that his intention, if elected, has always been to establish an official liaison office on Taiwan, much like the one established in Peking (and headed by George Bush in 1974-1975), rather than to give Taiwan formal diplomatic recognition. The American office currently on Taiwan is operated by a private foundation.

Bush said his intention was to use the trip to "articulate as best I can the objectives of a Reagan foreign policy," and he expressed confidence that those objectives would be "refreshingly received" in Peking.

Bush said, however, that Reagan's Taiwan policies probably would not be the main subject of discussion with Chinese leaders in Peking. The emphasis of those talks, he said, would be worldwide security and trade.

But the discussions also would deal with many other subjects, he said. "It's on the United States as a deterrent to Soviet aggression. It's on our commitment to trade. All those things Gov. Reagan enthusiastically supports."

Responding to questions about President Carter's attacks on him at the Democratic National Convention this week, Reagan said he supposed that "if you live in a fantasy land everything else looks like a fantasy."

Regan quoted a television commentator who remarked that the president was attempting to portray his opponent as "a combination of Ebenezer Scrooge and the Mad Bomber. He [the commentator] didn't think it would sell, and I don't think it will sell."

Reagan spent the week of the Democratic convention relaxing at his mountain retreat north of Santa Barbara. On Sunday he will begin a four-day cross-country campaign trip that will take him to Chicago, Philadelphia and Boston. He will address the annual conventions of the Veterans of Foreign Wars in Chicago and the American Legion in Boston.