The invention discloses a transmission device of an automatic sheet conveying mechanism. The transmission device comprises a fixed plate. A plurality of supporting plates are fixedly arranged on the front and back sides of the fixed plate. An upper movable frame is fixed to the upper portions of the supporting plates. A material containing frame is fixed to the top end faces of the supporting plates. A rodless air cylinder is fixed to the portion, below the upper movable frame, of the fixed plate. A stepping motor is fixedly arranged at the left end of the fixed plate. A driving wheel is fixedly arranged on an output shaft of the stepping motor. The left end and the right end of the upper movable frame are each fixedly provided with a connecting plate. The two ends of a movable screw rod are hinged to the corresponding connecting plates respectively. The left end of the movable screw rod stretches out of the connecting plates and isfixedly provided with a driven wheel. A transmission belt is tensioned on the driving wheel and the driven wheel. A rear sliding block is in screw connection with the movable screw rod. Clamps are fixed to the rear sliding block and a sliding block of the rodless air cylinder respectively, and the two ends of a sheet are clamped to the two clamps, so that the sheet is conveyed through synchronous operation of the rodless air cylinder and the stepping motor, and it is ensured that the sheet is in the tensioned state all the time.