The invention provides an automatic soft bag sealing device for core drilling. The automatic soft bag sealing device comprises a rotary drill rod, a core acquisition pipe and a soft bag. The core acquisition pipe is arranged in the rotary drill rod and the core acquisition pipe and the rotary drill rod are coaxially arranged. The soft bag is hung in the core acquisition pipe. The lower end of the soft bag is fixed to the lower end of the core acquisition pipe. A circle of groove is formed in the portion, close to an opening of the core acquisition pipe, of the inner wall of the soft bag in the circumferential direction. A memory alloy retainer ring is arranged in the groove. A constant temperature heater is arranged at the top end of the rotary drill rod and electrically connected with the outer side of the memory alloy retainer ring through a wire. The automatic soft bag sealing device for core drilling is used for sealing the bag after lunar soil is collected in the gravity-free environment, the soft bag is closed directly through the mechanical effect of high-temperature memory alloy, and the automatic soft bag sealing device has the advantages of being high in reliability, low in weight, simple in structure, convenient to operate and the like.