Former vice president Walter F. Mondale said yesterday that President Reagan should "admit the mistake" and ask Congress to undo some of the "unwise tax cuts" it approved last summer.

In a speech at Columbia University business school prepared and delivered before Reagan reaffirmed his present policies at a press conference, Mondale said that eliminating "expensive and egregious tax loopholes" and postponing "a portion of the later installments of the personal tax cut" would reduce deficits and allow the Federal Reserve Board to ease monetary policy and bring down interest rates.

In a briefing Monday afternoon, the prospective 1984 Democratic presidential contender said "I see no hope of a sustainable recovery from the current recession unless we take the pressure off the Fed" for tight-money policies. "To wait much longer before reversing course would be to take extreme risks."

Discounting administration claims that the Reagan economic plan has not been in place long enough to be judged fairly, Mondale said "it is a radical policy, and it has failed."

He added in the briefing that "the overwhelming portion of this the recession and the looming budget deficits is directly attributable to Reaganomics," not to the after-effects of the previous four years of Democratic rule.

"Within 12 hours of the adoption of these new policies, the stock market went through the floor and the bond market was in disarray," he said.

Mondale said he would like to see Congress immediately eliminate such "loopholes" in the new tax law as provisions for tax-credit-and-depreciation leasing and for easing the windfall profits tax.

He said the slowdown in scheduled personal income tax cuts should be timed so that it "does not conflict with antirecessionary efforts to revive economic growth." He left some doubt as to when he thought Congress should decide how much to scale back the tax cuts scheduled for mid-1982 and mid-1983.

The economics speech was the second in a series Mondale is making this fall. They put forth his basic views after what he has called a "period of study and reflection" following the 1980 election defeat that sent him, temporarily at least, into a Washington law practice.

He made a similar speech on foreign policy two weeks ago.

Asked by reporters to pinpoint changes from his earlier liberal philosophy, Mondale noted that he was not calling for any large-scale public works or public-service jobs program to combat the rising unemployment caused by the recession.

"The basic source of new jobs has to be in the private economy," he said, adding, "That is why we have to get interest rates down."

Mondale declined to criticize the Federal Reserve Board, saying that current monetary policy "may be a little hard," but cannot be eased until taxes are raised and deficits reduced.

For the long term, economic policy must emphasize public and private investment in technology and education, steps to encourage trade and exports, and conservation of energy, water and land, Mondale said.