Secretary of Labor F. Ray Marshall has promised inquiring congressmen that major improvements would be made soon in the workmen's compensation program for federal employees, which has come under strong attack for delays and insensitivity.

A letter from Marshall arrive in congressional offices Wednesday evening, and was an apparent effort to defuse criticism that was expected and was delivered at a hearing Thursday held by a House Education and Labor subcommittee.

The compensation program, administered by the Labor Department, is intended to pay federal workers for financial losses they suffer as a result of injuries suffered on their jobs. Similar programs are run by the states for employees of private firms.

Rep. Herbert E. Harris (D-Va.), one of several lawmakers who have been frustrated in attempting to intercede with the Labor Department in half of constituents - all from Alexandria - to Thursday's hearing, conducted by Rep. Joseph E. Gaydos (D-Pa.).

Darlene Snider, a secretary at the Internal Revenue Service, has three ribs broken when she was hugged tightly by an office "cut-up," she related. It took 13 months of unresponsive telephone calls and repeated visits to the compensation office at 666 11th St. NW, with much time lost form work, before she got the bulk of her money. "I just dropped it because I got tired of fooling with it," she testified.

Susan D. Patton, a clerk at the Arlington post office, suffered shoulder and neck injuries from handling heavy packages. She said she called the compensation office 60 times - once letting the phone ring 112 times - and got through only once. Even now, she said, "my file is lost . . . I can't get anybody even to talk to me."

Clarence S. Evans, a writer for the Navy Recruiting Command, fell while on the job in 1974 and had to leave his job permanently last December as an ultimate result of claimed injuries.

His dealings with the office were frustrating as he approached insolvency at the age of 51, he said, but he didn't blame the lower-level employees.

"The job, the program, is bigger than they are . . . I think there is some shortsightedness, lack of perception, at the top level," Evans said.

Evans contrasted conditions at the Labor Department with efficient, sympathetic action by the Civil Service Commission, which swiftly granted a disability retirement and made an emergency cash payment to him.

Marshall, who inherited the festering conditions when he took office in January, said in his letter that the compensation office's manpower remained stable during a recent five-year period in which claims increased 75 per cent.

Congress has authorized more employees, a survey has been undertaken and its recommendations received, and an automated data-procession system is being installed, Marshall said.

Gaydos said three more hearings would be held, including one to hear Labor Department testimony.