A new batch of tax benefits to encourage stock investments in business was proposed yesterday by a spokesman for the nation's mutual fund industry.

Investment Company Institute President David Silver suggested a plan under which an investor would have a tax deduction in the year an investment is made in stocks. Earnings on these investments would be sheltered from taxes.

As outlined by Silver, in an address to the trade group's annual meeting here, the stock investment proposal would be based on the same principles as Keogh plans and Individual Retirement Accounts.

Existing Keogh and IRA programs provide for tax deductions in the year that contributions are made and for tax-sheltered growth in separate accounts until retirement.

"All that would be needed would be to extend the benefits of these programs to all taxpayers regardless of whether they are self-employed or covered by other pension plans," Silver said.

The stock investment program could have maxiumum deduction of $7,500 a year (such as the Keogh plan) or be at a lower level. Whatever maximum amount is set, "I have little doubt that such a program would act as a mighty stimulus encouraging individual investment," the mutual fund spokesman added.

Despite changes last year in capital gains taxation, Silver contened, U.S. laws generally discourage individual investments in stocks. Included are purchases of shares im mutual funds that invest in common stocks.

Although Silver reported yesterday that mutual fund assets are now at a record level of more than $65 billion about one-third of that total reflects money market mutual funds that didn't exist a decade ago.

Money market funds have posted new highs in assets every day for the past several months as investors have sought a tie to interest yields for their dollars. But investment in stocks continues to sag, even in the mutual fund industry-which has performed twice as well as major market indices in the past five years on average.

"While this condition continues, and the public remains unwilling to invest in equities, the common stock mutual funds which still account for over half of industry assets will continue to languish . . . it augurs even worse for the country," Silver stated.

To back up his suggestion for new tax incentives. Silver pointed to recent decisions by Japan, Canada and France to allow such a course. Last July, France enacted legislation designed to channel savings into stock investments and permitting deductions of $1,150 a year from taxable income for stock purchases. Equities bought under the plan must remain in a custodial account for a least the four-year period that the program is in effect.

Silver said the U.S. Treasury had estimated the French plan would raise no more than 3 billion francs ( $700 million) by the end of 1978 but that by that time, 5 billion francs ($1.1 billion) were invested.