Sen. William Proxmire (D-Wis.) says he believes the Treasury is shorting the Social Security trust funds $2 billion a year by paying them extraordinarily low interest on Social Security cash reserves.

The Treasury has not responded formally, but officials, who asked not to be identified, clearly don't agree with the characterization, and say they fear changes could disrupt financial markets.

Proxmire, in a series of Senate speeches, has charged that the average rate of return on $47 billion in trust fund reserves invested in government securities in fiscal 1980 was only 8.3 percent, "scandalously low" at a time when government securities of various types were available yielding 13 to 20 percent.

The current system amounts to having the hard-pressed Social Security trust funds subsidize the Treasury, he said, calling it a "plunder" particularly unacceptable when Social Security benefit cuts are being proposed to solve financial pressures on the funds.

"The 8.3 percent average earnings on the $46.7 billion in the three trust funds in 1980 is a miserable showing. If any private trustee did that bad, he or she would be fired," Proxmire said.

Proxmire, with 17 Senate co-sponsors, has introduced a bill to add four members to the fund's board of trustees, which now consists of the secretaries of Health and Human Services, Labor and Treasury.

One of the new members would be an expert investment counselor. The bill also would require trustees to maximize the return on the fund and would change the formula for calculating interest to be earned by the trust funds on money placed in Treasury special issues.

Under existing law, Social Security reserves can be invested in a variety of government securities, including public issues commonly traded on financial markets.

But the Treasury puts the bulk into special securities issued only to the Social Security system. Unlike public issues, the special issues can be cashed in at any time at par. The interest rate is fixed by law, and is equal to the average market yield of all Treasury obligations (not just Social Security issues) that have at least four more years to run.

The formula has the effect of lowering what is paid on the special issues, since the older government securities on which it is based bear low interest rates prevalent at the time of issue.

At the end of fiscal 1980, the old-age trust fund held about $20 billion in special issues, many at only 7 1/8 to 9 percent.

Proxmire said there is no legal reason why the special issues could not be cashed in and invested in the more lucrative government securities freely available on the market, such as federal farm credit, bank board and housing issues, although, he added, "I wouldn't advise them to invest in Lockheed or Chrysler guaranteed instruments."

Many Treasury officials say they believe that Proxmire is overshooting the mark. For one thing, his figures are based on 1980, but today, new special issues are getting 13 to 14 percent and more.

Moreover, one official said, taking Social Security funds out of old low-rate special issues and plunking them into the market for federal farm credit and housing securities could drive up prices for those securities sharply.

When the securities are then sold, market prices might be depressed and Social Security might end up losing money on the deal.

Overall, officials said, the huge amounts of money involved could have a seriously disruptive effect on securities markets as a whole, and that was why the system of special issues was devised.

Proxmire, however, says he doesn't believe in these dire predictions. He said he will press to get his proposal into the pending Social Security financing bill.