With their big debate just days away, Vice President Bush spent only 90 minutes yesterday preparing for his encounter with Rep. Geraldine A. Ferraro (D-N.Y.) and the rest of the time being vice-presidential.

Bush's aides portrayed him as coolly going about his official business, conferring with Israeli Prime Minister Shimon Peres -- to the consternation of some advisers who worry that he should bone up more intensely for his debate with the first female vice-presidential nominee of a major party.

Still, like Bush, his aides minimized the debate's political importance. "They shouldn't put pressure on their candidate like that," Bush's press secretary, Peter Teeley, said reprovingly of Democratic strategists, who contend that the ante for the Bush-Ferraro debate is much higher because of Walter F. Mondale's strong showing in Sunday's presidential debate.

Thursday's vice-presidential debate has taken on new significance. Republican campaign advisers say a clear victory by Ferraro could make President Reagan and Bush look to voters like a "losing team." And despite tones of nonchalance, there are signs that this has created unease in the Bush entourage.

One such sign came Monday when Bush's wife, Barbara, in an uncharacteristic slip, called Ferraro "that $4 million -- I can't say it but it rhymes with rich." She made the remark after complaining to reporters that Mondale was "unfair" in Sunday's debate for making an issue of her husband's income taxes.

Mrs. Bush called Ferraro to apologize, and Ferraro accepted. Yesterday, Mrs. Bush said the word she was thinking of was "witch."

On the surface, Bush has every reason to be confident about the showdown. He has far more experience than Ferraro -- as the globe-trotting incumbent and as a former ambassador and CIA director. And his rating in the polls is far higher than hers despite her greater success at drawing crowds on the road.

Bush said in an interview that his strategy for the debate is to "be yourself" and "to make clear the differences between our plan for the future and theirs" -- in Bush's view, between the era of prosperity ushered in by President Reagan and the Mondale program of "doom and gloom" and "tax and spend."

But Ferraro's status as a female vice-presidential contender and her feisty, go-for-the-jugular style make her a wild card who could foil Bush's best-laid plans.

Republican advisers say they fear that she could unnerve Bush and force him to reveal a high-strung side that has hurt him during his career -- most notably in a debate with Reagan in Nashua, N.H., in 1980. That standoff put Reagan on the road to the presidency and helped knock Bush out of the race.

Bush, who drew relatively little attention in his first three years as vice president, has become testy during the 1984 campaign over press scrutiny of his differences with Reagan in 1980 on abortion, the Equal Rights Amendment and "voodoo economics" -- all issues Ferraro is likely to press.

Furthermore, Bush's friends point out, the vice president is a gentleman who is not always comfortable on the attack.

To prepare for Ferraro, Bush has practiced debating Rep. Lynn M. Martin (R-Ill.), who resembles her Democratic colleague in many ways -- from her blond hair to her tough-talking, fiery style.

A Republican source said Martin has "thrown everything she has" at Bush to teach him to respond to a female attacker. The vice president reportedly said after the first session that Martin had won Round 1.