President Reagan, explaining why he acted administratively to head off legislation imposing sanctions on South Africa, said yesterday that he would not have had to do so if Congress had approved line-item veto authority for the president.

This raised eyebrows on Capitol Hill because none of the pending veto measures, including a Senate bill embraced by the administration, would apply to anything other than appropriations bills.

Currently, a president must veto an entire appropriations bill in order to block a single spending item. Line-item veto legislation would allow him to veto individual items without rejecting the whole bill.

A White House spokesman acknowledged that Reagan's proposal for line-item veto authority would not have covered the South Africa sanctions bill. However, the spokesman said there were parallels because Reagan objected to certain parts of the sanctions bill while supporting others.

"He meant to imply there were problems with the sanctions bill much like appropriations bills, where there may be things he likes and things he doesn't like," the spokesman said.

Reagan's comment on line-item veto authority came when he was asked why he would veto the sanctions bill in light of its similarities to his executive order. ". . . This wouldn't have been necessary if I had what a president should have, which is line-item veto. I could have signed the bill and line-item vetoed out . . . ," he said.

A bill to give the president line-item veto authority for a two-year trial was shelved in the Senate during the summer after proponents failed in three attempts to end a filibuster against the measure.

Backers of the proposal say they hope to bring it to a vote again this fall, possibly in connnection with debt-ceiling extension legislation.