President Reagan filled another ambassadorial post yesterday, choosing Ernest Henry Preeg to be U.S. ambassador to Haiti. Assuming the Senate approves the choice, Preeg, a career foreign service officer with extensive experience in the Caribbean, will succeed Ambassador Henry Kimelman.

The administration also is considering installing a career diplomat as ambassador to the People's Republic of China. The reported choice: Arthur W. Hummel Jr., a 30-year diplomatic veteran now serving as ambassador to Pakistan. Hummel, born in China of American parents, speaks Chinese and studied in China for a year, until the day after Pearl Harbor, when he was put in a Japanese internment camp. He escaped three years later and joined a unit of Chinese guerrillas until the end of the war.

Hummel also has served as ambassador to Burma and Ethiopia, and did a couple of stints in Washington during the Nixon and Ford administration.

The ambassador's spot in Peking has been vacant since President Carter's appointee, Leonard Woodcock, resigned late last year. The post has been held by non-career appointees in recent years, but the Reagan administration hasn't completely sorted out its policy toward China, and the State Department is said to have urged successfully that a career diplomat be named to head the embassy there.