The Reagan administration will take a final shot this year at an important, but nearly invisible, civil service fringe benefit: the virtually automatic 3 percent longevity raises that about one-fourth of the federal work force gets every year in addition to any general pay increase.

In place of within-grade raises, which go to workers with sufficient time in grade and the equivalent of a satisfactory performance rating from their boss, the White House wants Congress to authorize a merit pay system. It would give bosses more leeway to give bigger, more frequent raises to top performers, regardless of longevity.

Most pay grades have 10 steps. The pay differential from the bottom to the top of each grade is about 30 percent, and it takes a worker who remains in one grade about 18 years to reach the top of a grade.

Currently federal workers qualify for within-grade raises after they have been in steps one, two or three of their grade for 52 weeks; after 104 weeks in steps four, five and six of their grade, and every 156 weeks in steps seven, eight and nine. Those at the 10th step get no step raises. Nor do supervisors and managers covered by the GM (merit pay) system set up by the Carter administration.

The Office of Personnel Management says that 99 percent of the employees who qualify for the step increases by virtue of time in grade get them.

Last year as part of a budget-cutting plan Congress considered, and rejected, a proposal to freeze within-grade raises for a year. The administration wants to abolish them altogether, but it is unlikely to sell that idea to the Democratic-controlled Congress.

Redskins Miniholiday

The president told Constance Horner, head of the Office of Personnel Management, to encourage bosses to grant administrative leave to Washington area federal workers from 11 a.m. to 1 p.m. tomorrow, so that most of the area's 350,000 civil servants can watch the Super Bowl winners parade downtown.

Reagan granted leave for a similar Super Bowl parade here in 1983. Time off probably will not go to postal employees because they are not under direct presidential control.

An aide to Rep. Patricia Schroeder (D-Colo.) said he doubts anyone in Denver had even considered releasing that town's 35,000 federal workers for an event to honor the Redskins.

Presidents' Summit

Steve Morrissey, the National Association of Retired Federal Employees president, met with President Reagan last week to talk about ways to improve the public image of government workers.

Morrissey made a pitch for a presidential proclamation honoring public servants.

He also signed Reagan, who is a year away from retirement, as a NARFE member.

Federal Personnel Guide

The 10th annual edition of the easy-to-read handbook of federal facts and figures is hot off the presses.

The 146-page book ($4, which includes postage) has special information on the old and new retirement systems and a new section on the U.S. Postal Service. For information write FPG c/o P.O. Box 274, Washington, D.C., 20044, or call 656-0450.