Secretary of State Alexander M. Haig Jr. flew into Peking today hoping to "restore mementum" to closer U.S. relations with the People's Republic of China and to avoid public disputes about Taiwan.

The famous "senior official" who speaks authoritatively for secretaries of state, in a briefing for reporters in his traveling party in Hong Kong yesterday, went to extraordinary lengths to steer clear of the contentious and potentially damaging subject of U.S. arms sales to Taiwan. In recent days, a drumfire of official statements from Peking has attacked the Reagan administration on the issue.

Haig, the official said, expects to place heavy emphasis in his talks with the Chinese on a strategic "convergence of views" in opposition to the Soviet Union and Vietnam.

Weighing his words carefully, the senior official said Haig will tell Chinese leaders that "we are going to continue to manage our unofficial relationships with the people of Taiwan as we have since normalization" of Sino-American diplomatic relations in January 1979.

Before leaving Washington, the same top State Department official told reporters in a briefing that U.S. arms sales to Taiwan have totaled $700 million to $800 million per year since normalization of relations with Peking and said, "I anticipate that there's not going to be a cessation of it."

The Taiwanese have been pressing for several large new purchases, including an advanced jet fighter and shipboard missiles. Peking considers Taiwan a wayward province with no separate political legitimacy.

While Haig was en route to Peking, the president's daughter, Maureen, was in Taipei on behalf of a private association promoting U.S. exports. Reagan's trip was described by the senior official here as "unrelated in any way'' to Haig's mission and "not at all" a problem.

The senior official went out of his way today to condemn Vietnam as a Soviet "proxy" and referred to the Vietnamese -- in language often used by China's leaders -- as "Southeast Asian Cubans."

"One cannot but be increasingly concerned" about Hanoi's policies, he said. He listed as examples of these policies the "blatant violation" of the Nixon administration's 1973 Paris accords, which Haig helped to arrange, the Vietnamese invasion of Cambodia and a recently growing Soviet military presence, including Soviet naval use of Camranh Bay and Soviet intelligence collection facilities at the same U.S.-developed port.

He said Vietman was increasingly isolated in the international community because of its actions and left no doubt that the Reagan administration aims to continue and even deepen Vietnam's isolation.

The activities of Vietnam, described as "indirect Soviet pressure" against China, were cited in justification of U.S. interest in improving Chinese military strength. Haig is prepared to discuss the "broad framework" of Chinese arms requirements with Defense Minister Gene Biao in Peking, but he does not anticipate detailed talks on specific weapons, reporters were told.

The United States hopes, the official said, that China and noncommunist Southeast Asian nations will take the lead in opposing Vietnam.

China, considered in the 1950s and 1960s as America's most implacable foe, was by the closing months of the Carter administration officially regarded as "a friend" but not yet an ally.

The senior official's briefing today carried the description a half-step forward by calling China "a friendly non-aligned country with which strategic relations are a strategic imperative." The official said President Reagan has "a firm commitment" to advancing this strategic relationship.