More than 81,000 retired United Mine Workers members will receive their regular pension checks next month. But their retirement income is uncertain beyond January, because of a nationwide coal strike.

A spokesman for the UMW health and retirement funds said Tuesday the union's battered 1950 pension fund has only enough cash to meet its one-month obligation of $20 million.

"We don't know what will happen in February," said Barbara Moldauer.

The pension plan, covering miners who retired before the end of 1975, is financed by contributions from coal companies. The UMW has been on strike against the Bituminous Coal Operators Association since the expiration of its old contract at midnight Dec. 6, and no contributors are made to the funds when coal is not mined.

The fund has been drained of reserves since summer, partially because of wildcat strikes. It has depended on company contributions from one month to pay pensions for the next.

Negotiations on ending the strike, now in its third week, have been draging on in Washington, and it is unlikely that any more union coal will be mined this month or early in January.As a result, the pension fund is unlikely to get an infusion of cash for several weeks.

Bargaining subcommittees met Tuesday to discuss miner benefits, health and safety and worker grievances for a new contract. The meetings were expected to continue yesterday.

A key union demand is that the funds be replenished.

The depleted pension plan normally provides eligible retirees with monthly benefits of $250. Retirees who qualify for black lung payments under a government program receive smaller pension checks of $225. But their black lung payments range from $219 a month for a single retiree or survivor to $440 a month for a retired miner or survivor with three dependents.

In addition, many retired UMW members or dependents qualify for Social Security benefits.

In a letter prepared for mailing later this month with the pension checks, trustees of the UMW pension funds wrote, "The enclosed check represents payment of the full amount of this month's pension. But we are not sure what will happen next month and afterwards . . .

"As matters now stand, there will not be any money to pay your February pension. And even if coal production resumes in January, it cannot be assumed that there will be enough money in the trust by the beginning of February to pay all or part of your pension."