Secretary of State Alexander M. Haig Jr. will inaugurate the Reagan administration's top-level contacts with the People's Republic of China by visiting Peking next month, the State Department announced yesterday.

Haig's visit to China of about three days is expected to follow his previously announced meetings in Manila with the foreign ministers of the Association of Southeast Asian Nations June 19-20 and in Wellington, New Zealand, with the foreign ministers of Australia and New Zealand June 22-23.

En route home, Haig plans to visit Tokoyo to brief the Japanese on his discussion in China.

The Sino-American relationship in the Reagan administration remains a question mark in several respects, especially because of the president's previous advocacy of closer U.S. ties to Taiwan.

The new administration, responding to advice from Haig, decided in its early weeks to make no basic changes at that time in the Peking and Taiwan relationships, and to defer action on Taiwan's request to purchase advanced military aircraft.

These decisions were described as preliminary, pending a full-scale review of China policy.That review has not yet been completed, according to administration officials.

Questions of security relationships and arms sales to Peking, as well as those to Taiwan, await decisions at authoritative levels of the administration.

In recent weeks the United States has been tacitly aligned with Peking in supporting a non communist "third force" of Cambodians who are resisting Vietnamese occupation of Cambodia.

A leader of the "third force" group, former Cambodian prime minister Son Sann, received political support from the administration on a recent trip to Washington. There have been reports that his followers have received military as well as political support from Peking.

State Department officials said yesterday that the United States has appealed the UNICEF and other international agencies to continue providing food to Son Sann's civilian followers, but that no direct U.S. food aid is planned for them.