Teamsters President Frank E. Fitzsimmons formally resigned as a trustee of his union's biggest pension fund yesterday, climaxing months of government effort to rescue the fund from what officials regard as mismanagement of its assets.

Resignation of Fitzsimmons and three other longtime trustees fulfilled an agreement negotiated under heavy pressure and threats of court action from the Labor Department.

The government agreed to drop plans for legal action and allow the plan to keep a disputed tax exemption it needs to operate. In exchange, control over the fund's assets will be turned over to independent financial managers led by the Crocker National Bank of San Francisco and new trustees will handle the rest of the fund's business, including distribution of benefits.

Labor Secretary Ray Marshall hailed the resignations and said he thinks the plan is now back on track.

The Central States, Southeast and Southwest Areas Pension Fund, as it is formally named, was created in 1955 by James R. Hoffa, who went to jail in 1967 for defrauding it.

It has been the target of charges of mismanagement and corruption almost ever since, but has eluded decisive government action until now - and some congressional critics and Teamster dissidents question whether the purification process is yet complete.

Central States is the largest of 270 individual Teamster pension funds and one of the biggest pension investment pools in the country, with about 450,000 participants in 33 states and declared assets of $1.4 billion.

Each month it takes in about $30 million in employer contributors of up to $100 for each of 380,000 working Teamsters. With monthly payments of about $20 million in pensions to more than 70,000 retirees, ranging from $90 to $550, the fund each month has about $10 million left over for administrative costs, investments and setasides against financial loss.

Although the fund has been under investigation of one kind or another for much of is 22-year history, key facts about it remain a mystery - including its exact worth.

While the supposed value of its assets is $1.4 billion, its portfolio is ridled with bad or questionable real estate loans, often made to organized crime figures or others for whom it is the "bank of last resort," according to government investigators and lawyers.

Hence the real value of the fund - meaning the investments it can count on to finance future pension benefits - is considerably lower than $1.4 million, these officials say.

They say the fund has conceded that more than 10 per cent of its investment assets have gone sour and only last month the fund took the first step toward foreclosing $47 million worth of loans to companies controlled by San Diego financier Alen R. Glick, whose holdins have reportedly benefited from $146 million in Central States loans over the years.

The officials say no one will really know the value of the fund until an independent real estate audit is made, as contemplatedunder the agreement with the Labor Department.

No one is saying that the fund is in danger of financial collapse, although it has conceded that it, like many pension plans, is underfinanced. What the government contends is that the fund's loan practices - including its heavy investments in real estate - violate minimum requirements for "prudent" investment practices under the 1974 Employee Retirement Income Security Act (ERISA).

Real estate loans are normally shunned as too risky by pension investors, but they constitute the bulk of the Teamsters' portfolio.

The fund has had up to 89 per cent of its assets in real estate - principally housing developments, hotels, motels and office buildings but also race tracks, gambling casinos, nursing homes, cemeteries, and a cathedral for a television evangelist. Some were second and third mortgages.

Under government pressure, the fund ceased making new real estate loans in March, 1975. But even by the fund's own calculations, 60 per cent of its assets are still in mortgages and real estate loans, rather than such things as stocks and bonds. The average for all pension funds last year was less than 5 per cent in real estate, according to the Securities and Excange Commission.

The government also charges that the fund has made many loans without rudimentary financial precautions.

"A guy would come in and say he wants to build a resort in the desert that will cost $50 million and he wants $40 million," said a government attorney who has worked on the case "He says it will be profitable and yeild a 10 per cent return. The trustees might take a plane out to the desert and look at the land. Then they'd say, 'OK, give it to 'em. That would be the end of their checking."

The fund has been a major well-spring of Teamsters' power over the years. "They give loans to friends but they also put their money where it will do good politically,? said an official by way of explaining a possible reason why the put $23 million in the National Bank of Georgia last year at about the time that Jimmy Carter emerged as the frontrunner for the Democratic presidential nomination. Carter had loans from the bank, which was headed by his close friend, Bert Lance, now head of the Office of Management and Budget.

If the Teamsters had hoped to curry favor with the Carter crowd, they appear to have failed.

Under prodding from Congress, Marshall took personal command of the department's more than year-long investigation of the fund and threatened to sue the trustees if they failed to relinquish control of the fund's assets to independent managers.

But the step-by-step negotiated settlement that culminated in the resignation of Fitzsimmons does not guarantee changes in the administration of pension benefits, which will remain in the hands of the successor trustees.

This is important because of repeated charges - denied by fund trustees - that the fund disqualifies large numbers of benefit applicants for technical and arbitrary reasons, both to save money and maintain discipline.

Daniel Shannon, director of the fund, testified recently that the fund has a rejection rate of 7.4 per cent, which he described as lower than other plans'.

PROD, a leading dissident Teamsters group, contends that the figure does not reflect many qualified beneficiaries who are informally rejected by local unions officials and never take their case to the fund.

PROD and some congressional Teamster watchdogs complain that the agreement doesn't go far enough because it retains the fund's control over benefits and contemplates cessation of the government's investigation into assets management.

They say the Labor Department should have hauled the trustees into court, seeking receivership or some other method of removing the fund entirely from Teamster control and forcing restitution of lost funds.

Department attorneys respond that court action might have been too slow, leaving the fund open to mainipulation until the legal issues were resolved.