South Korea's most experienced career diplomat was named yesterday to become ambassador to the United States, at a time when his country's activities here are the subject of wide-ranging federal investigations.

Kim Yong-shik, 63, a former foreign minister and U.N. ambassador who is currently the South Korean ambassador in London, will replace Hahm Pyong-choon in Washington in about a month, an embassy spokesman here said yesterday.

The foreign ministry official who made the announcement in Seoul gave no reason for the change. But observers here view Kim's appointment as an effort by South Korean President Park Chung Hee to make a fresh start with the Carter administration.

During the past several months a Justice Department inquiry, newspaper headlines and now at least two congressional investigations have focused attention on allegations that Park personally directed a lobbying campaign to buy influence on Capitol Hill.

Hahm, a noted South Korean scholar before he joined the diplomatic service in 1970, has been ambassador here since early 1974. He will become ambassador at large.

An embassy spokesman noted that his rotation could be considered routine after a three-year tour of duty in Washington. Hahm's predecessor, however, served here for six years.

Kim has been in his country's foreign service since 1949. His most recent assignments have been as South Korea's ambassador to the U.N. 1964-70: special assistant to President Park for foreign affairs, 1970-71: minister of foreign affairs, 1971-73 and ambassador to Great Britain since 1974.

Donald L. Ranard, a State Department Korean specialist in the early 1970s and a longtime acquaintance of Kim said yesterday, "He's their most sophisticated and urbane diplomat. It seems he's being sent here during a difficult time to make a good impression."