The utility model relates to a draining system for an air bag of a pressure fan applied to the field of coal mine machines, in particular to an automatic draining device for an air bag. The automatic draining device for the air bag comprises a water outlet pipeline and a branch pipeline, wherein the branch pipeline is arranged in the middle of the water outlet pipeline and is blind pipe which is vertically placed, the top of the blind pipe is closed, a floating ball is arranged inside the branch pipeline, one side of the branch pipeline is communicated with the upper end and the lower end of a water level display tube which is vertically placed, a limiting control valve is installed on the top of the branch pipeline, a cylinder is fixedly installed on the other side of the branch pipeline in the direction parallel to the branch pipeline, one end of a piston rod of the cylinder is connected with a control ball valve through a linkage mechanism, the control ball valve is arranged on the water outlet side of the water outlet pipeline, and a water inlet of the water outlet pipeline is connected with a water outlet of the air bag. The automatic draining device for the air bag further comprises a pneumatic control circuit. According to the automatic draining device for the air bag, draining efficiency is improved, and labor is saved; meanwhile, the situation that detection and drainage are not conducted in time due to human factors, and thus equipment is damaged is avoided.