With Archbishop Desmond Tutu lending his support, Senate opponents of South Africa's apartheid government prepared to introduce legislation Thursday that would virtually cut off economic ties between the United States and Pretoria.

The bill, similar to one pending on the House calendar, would force U.S. businesses to divest their holdings in South Africa within a year and would ban most trade with that country."It is time for Congress to take action again," said Sen. Edward M. Kennedy, D-Mass., a sponsor of the bill. "So long as apartheid continues, American business has no business doing business with South Africa."

Other supporters include Sens. Lowell Weicker, R-Conn., Alan Cranston, D-Calif., Paul Simon, D-Ill., and Carl Levin, D-Mich.

Tutu, a 1984 Nobel laureate for his fight against racism, held a series of appearances and meetings on Wednesday to press his message that the longstanding U.S. policy of diplomatic pressure on South Africa has failed to produce substantial reforms. He urged support for new and tougher economic sanctions.

While supporters of the sanctions are optimistic about House passage, the bill's fate is unclear in the Senate, in part because Republicans who were key to the passage of limited sanctions in 1986 are reluctant to go along with harsher measures.

A congressional study issued this week showed South Africa's exports to 24 countries which report trading data to the International Monetary Fund decreased by 7 percent and imports plummeted by 31 percent from 1982 through the first half of 1987. The declines came while overall trade was rising substantially.

The interim report by the General Accounting Office did not say how much of the decline was due to the limited sanctions imposed by Congress in 1986 over President Reagan's veto.

The report also said U.S. direct investment in South Africa declined by about 10 percent since 1982.