A House Labor subcommittee opened hearings yesterday on a plan to adjust automatically the minimum wage level each year.

Since enactment of the 1938 law setting a wage floor that must be paid covered workers, the figure has been fixed by act of Congress and then remained the same for several years until Congress was ready for another employer-labor fight and raised it again.

The new plan by subcommittee Chairman John H. Dent (D-Pa.) is to tie the minimum to the average wage paid manufacturing workers. His bill would set the minimum at 55 per cent of the average wage 30 days after enactment and at 60 per cent the first of next year. Each year it would automatically be adjusted to stay at 60 per cent.

The effect would be to raise the minimum from $2.30 to $2.85 within 30 days and to about $3 the first of next year. It would also repeal the differential for lower-paid farm workers next year.

The AFL-CIO endorsed the automatic escalator yesterday, but asked that the minimum wage be raised to $3 immediately. Labor lobbyist Andrew J. Biemiller said the late Sen. Robert A. Taft - known as Mr. Republican - had proposed that approach in 1949.

About 6 million workers who are paid no more than the minimum wage would benefit immediately if it is increased.

Dent has discussed the automatic adjustment formula with President Carter and Secretary of Labor Ray Marshall. But the administration has not taken a position on it.

Two liberal Democrats on the subcommittee, second-termers Paul M. Simon (D-Ill.) and George Miller (D-Calif.), indicated they might consider permitting a teenage differential - paying youth less to perform the same work as adults - because of the high teenage unemployment rate.

Simon said a minimum wage bill won't pass unless it does something to help young differential for fear employers would lay off heads of families and hire teenagers to do their jobs at less pay.

The American Hotel and Motel Association opposed a provision in the bill to repeal existing law that permits them to pay less than the minimum wage to waitresses and other workers who receive tips. The association said the added costs would require hotels to increase rates by 75 per cent or lay off more than 200,000 workers.