The Chinese are preparing to celebrate their National Day with so many prominent American Republicans on hand that Tienanmen Square looks a little like the Capitol Hill Club at lunch.

For the last two nights, thousands of marchers carrying red banners have jammed the square, while spotlights played patterns in the sky, all in rehearsal for the celebration Sunday of China's success in surmounting the earthquake and political upheavals of the past year or so.

Watching the massive spectachle as guests of the government are former Secretary of State William P. Rogers, George Bush and Dean Burch, both former Republican national chairmen and official of the Nixon and Ford administrations, and James E. Baker III, who managed former President Ford's 1976 campaign.

Bush, Burch and Baker arrived here late Sunday in a 23-person group that also included 85-year-old broadcaster Lowell Thomas, who is determined to celebrate his second honeymoon in Tibet, where the party is scheduled to go Friday.

Rogers, who was last here with former President Nixon on the historic journey in 1972, arrived today from Tokyo on a trip that is part business and part tourism Thursday, another prominent Republican, the former Pennsylvania governor and Ambassador to the United Nations, William W. Scranton, is due to join Rogers.

Tonight, Rogers, Bush, Burch and Baker gave a banquet for their Chinese host group, tended by Hao Tehching, the president of the Chinese People's Institute of Foreign Affairs. Hao's toast was so studded with references to the "statesmanship" of Nixon and Rogers that it seemed almost an unspoken rebuke to President Carter and Secretary of State Cyrus Vance, who were not mentioned at all by the Chinese officical.

But the oratorial highlight of the banquet was provided by Thomas, the retired broadcaster who is waging a not-so-subtle war of wills with Chinese officials over his desire to return to Tibet.

Thomas, accompanied by his bride of a few weeks, wants desperately to go back to Tibet, whose cause he championed at the time the Chinese took over the country.

The Chinese say they are concerned about Thomas' age and the fact that he wears a pacemaker to correct a slight irregularity in his heartbeat. But in a toast tonight, the veteran broadcaster pointedly said, "I happen to have been the mountaineer all my life, so when I go to high altitudes, I feel like I'm going home."

Thomas noted that he and his bride, Marianne, had traveled to the 17,500-foot level on Mt. Everest on their recent honeymoon, and he said, "Now I'm going to take her on a second honeymoon to Tibet."

He and other members of the Bush party are scheduled to learn Thursday whether they have passed their physical exams for the trip to Lhasa, the Tibetan capital.

Bush, who was the head of the U.S. liaison office in Peking for 14 months in 1974 and 1975, was invited back by Chinese. The group he has assembled includes Burch, the 1964 Republican national chairman and later chairman of the Federal Communications Commission and counselor to Nixon and Ford, and Baker, the Houston attorney who shifted from under-secretary of commerce to the top spot in last year's Ford campaign.

Rogers was invited separately by the Chinese, and his arrival in Peking at the same time is coincidental.

But the sense that Republicans still control the Peking outpost, despite last November's election results, is heightened by the fact that Leonard Woodcock, the former United Suto Workers president who was named by Carter to head the liaison office, is absent from China this week on a trip to the Untied States.

Bush received an unusually warm reception, not just from Chinese officials, but also from his former colleagues in the diplomatic corps and the liaison officer staff.

He and his party had a 90-minute meeting yesterday with Vice Premier Teng-Haiao ping. Reporters, who were allowed in only for the first minutes, heard Teng tell Bush that he was welcome in China at any time. "Even when you were the head of the CIA," the job Bush took after leaving Peking late in 1975. "We have nothing to hid," Teng said laughing.

Today Bush and anothe member of his party, J. Hugh Liedtke, chairman of the board of Pennzoil, met with Chiang, the minister of foreign trade.

Bush declined to describe either conversation, but other officials said there were no indications of any shifts in Chinese policy on trade or other matters.