The present disclosure discloses a pressure-retaining cap screwing and unscrewing device for a marine natural gas hydrate pressure-retaining rock-core barrel, wherein it comprises a cap-unscrewing chuck, a support cylinder, a pressure-retaining cap screwing and unscrewing head, a spring, a screwing and unscrewing shaft, and a pressure-retaining cap screwing and unscrewing sleeve, a reducer and a motor; an input shaft of the reducer is connected to the motor, an output shaft of the reducer is the screwing and unscrewing shaft which is a tubular structure with a plurality of guide holes provided on a side wall thereof. The device is simple in structure, compact, light in weight, and convenient to control, and the pressure-retaining cap can be discarded.