Congress plans to move quickly on a new federal retirement plan, tentatively approved yesterday by the White House, to cover the 400,000 federal workers hired since January 1984. One of the first orders of business is to get federal agencies, which began deducting an extra 5.7 percent from the paychecks of new hires on May 1, to stop making the deductions.

The proposed system, modeled after private-sector plans, bases benefits on Social Security, a modified civil service pension and earnings from a voluntary, tax-deferred investment plan. If it is approved by the House and Senate and then by the president, it will go into effect next January.

Under an interim system set up by Congress more than two years ago, new workers paid the full Social Security tax but contributed only 1.3 percent of their salaries toward retirement.

Feds hired before 1984 pay only the Medicare portion of Social Security, but contribute at least 7 percent to the retirement fund.

The agreement covering new hires expired May 1, after the White House objected to the compromise plan's cost and early retirement features.

Since May 1, those new employes, about half the members of Congress and all political appointees, have had to pay full Social Security and civil service contributions amounting to more than 15 percent of their salaries.

Under the new retirement system, fully covered employes could invest up to 10 percent of their salaries in a tax-deferred thrift or investment plan. The government would match up to about 4 percent of that amount for each worker.

Employes under the present retirement system could join the investment plan, but would not get the government match.

Contributions and earnings in the investment program would be tax-deferred until employes retired. It would be subject to any limitations enacted as part of tax reform.

Workers under the new system could retire at age 57 after 30 years' service but with financial incentives to encourage them to work until at least age 62. Under the current retirement system, the earliest that most employes can retire is age 55 after 30 years' service.

Congressional sources said they hope to work out an arrangement that will spare new employes from having to pay the extra 5.7 percent deduction much longer.

The extra deduction already has been programmed into many agency computers. Checks reflecting the new deductions will begin to show up this week and next. No Butts About It

Effective today, on-the-job smoking is being banned at the Merit Systems Protection Board headquarters at 1120 Vermont Ave. NW. The ban applies to offices, reception areas, restrooms and hallways on the agency's two floors.

While many federal offices have no-smoking areas, the merit board's ban is believed to be the first agency-wide order.

Maria L. Johnson, acting chairman of the board, said that there will be a 90-day grace period when employes can leave the board's turf to catch a quick puff. About half the board's 325 staff work at the headquarters.

The board will allow administrative leave for any workers who take smoking cessation classes given by the American Cancer Society. Part Timers' Workshop

The Association of Part-Time Professionals will hold its seventh annual training workshop today at the Commerce Department auditorium. Federal personnel officials and congressional aides will talk about employment opportunities and the benefits outlook for part-time workers. Registration is at the door. For information, call 734-7975.