Former vice president Walter F. Mondale said yesterday that President Reagan's economic and budget policies are ravaging the housing industry and will result in a complete drought of affordable housing for the poor and the middle class.

Mondale said that high interest rates encouraged by Reagan's policies, and the abandonment of a prior federal policy to aid construction of low- and middle-income housing, have combined to make housing unaffordable for large numbers of Americans and to depress a sector of the economy severely.

"In my opinion, it is the worst economic policy in America's history. It doesn't make any sense," said Mondale, speaking at the opening of the annual convention of the National Housing Conference (NHC), an umbrella organization for a wide range of housing groups from home builders to public officials and savings and loan officials.

Reagan's economic policy has spawned "devastating interest rates that have savaged the housing market. The bottom line is no housing," Mondale said.

The NHC convention opens at a time when the housing industry is one of those suffering most from the current recession. Interest rates that have lingered in the upper teens since late 1979 have made housing affordable to only about 5 percent of the nation's prospective buyers, thrown some realtors out of business and crippled the lumber and furniture industry.

Mondale also told the audience of about 150 that the president has reversed a long-standing federal housing policy emphasizing housing construction to increase the supply of shelter for low- and moderate-income families.

Instead, Mondale said, the administration intends to increase only the demand for more housing through a new program of giving housing vouchers to some poor families who would then compete for housing in the open market.

The voucher program, along with the administration's plans to build no new subsidized housing, increase rents in existing subsidized housing and count food stamps as cash income in the calculation of the amount of rent to be paid by the tenants, represents an abandonment of the nation's 30-year-old commitment to decent and affordable housing to all its citizens, Mondale said.

Currently families who live in subsidized housing pay only 25 percent of their income as their rent. Under the proposed budget for fiscal 1983, the cash value of food stamps would be counted as cash and included in that 25 percent computation, so that the more food stamps a family receives, the higher the family's rent payment.

U.S. Housing and Urban Development Secretary Samuel R. Pierce Jr., in announcing his new budget last month, described it "as a major departure from the past," but added it is necessary "if we are to continue providing any housing assistance to needy families."

The budget proposes reducing the number of new and rehabilitated subsidized housing units from 110,231 in fiscal 1981 to 22,812 this year, and reducing it further to 10,000 in 1983. Those 10,000 units next year would be exclusively for the elderly.