President Carter urged Congress yesterday to give small depositors the same return on their money as large investors.

Carter, in a special message to Congress calling for a broad set of financial reforms, said it was "unconscionable for the federal government to prohibit small savers from receiving the return on their deposits that is available to large and sophisticated investors."

Federal law currently limits the amount of interest that banks and other savings institutions can pay depositors. The limits have been stuck at about 5 and 5 1/2 percent for savings accounts.

With inflation driving up interest rates in the markets however, those with a minimum of $10.000 to invest can earn as much as 9 1/2 percent on their money by buying corporate securities or government securities. There is no ceiling on the amount of interest that can be paid on these.

Consequently, Carter asked Congress yesterday to permit "an orderly transition to a system where the average depositor can receive market-level interest rates on his or her savings."

Carter declared that the existing ceilings "are costing the American people billions of dollars in lost interest annually." Senior citizens and those who keep most of their money in passbook accounts have suffered the most he said. The president also noted that current limits have contributed to sharp fluctuations in the availability of housing credit. He said the proposed visions would "help assure a steadflow of mortgage credit for homeowners."

Carter asked Congress to:

Phase in changes that would allow interest on deposits to market-rate levels, subject to "emergency action" by regulators to protect the financial soundness of financial institutions and the operation of monetary policy.

Grant power to offer variable-rate mortgages to all federally chartered savings institutions. On such home mortgages, interest rates would rise or fall with market rates.

Permit all federally chartered savings institutions to invest up to 10 per-cent of their assets in consumer loans. They now are not allowed to make any consumer loans.

Allow all federally insured institutions to pay interest on individual checking and check-like accounts.

The president provided only a general outline of his overhaul package.Reporters were told later at a briefing by a high Treasury official that specific recommendations - along with a detailed task force report on the subject - would be sent to Capitol Hill within the next few weeks.

Changes in the nation's financial rules have been the subject of a handful of presidential study groups over the past 10 years, without any substantial result. But administration officials claimed yesterday that a change in the nation's economic climate has improved the chances for some reform.

Banking and consumer groups yesterday were quick to endorse the president's proposals. But the savings and loan associations were highly critical of the plan, and it is from this group that the main opposition is likely to come.

Savings and loans associations now can offer depositors a quarter of a percent more interest than banks - a relationship established in 1966 to help channel household savings into home mortgages. Carter's package would eliminate this differential. The U.S. League of Savings Associations termed the president's plan "a devastating blow to housing," fearing it would eliminate some S&Ls.

Antitipating opposition from the savings institutions, Carter included in his statement a pledge "to protect the long-term viability of savings institutions" so that they can "continue their traditional role in meeting our nation's housing needs."

The president also stressed that the transition to a less-regulated system should be gradual, though he was not specific. Proposals by industry groups and legislators have called for phaseins of from five years to much longer terms.