An automatic water discharge device is provided for use with a pneumatically operated apparatus and includes a housing including a water accumulation chamber; an electromagnetic valve coupled to the water accumulation chamber and including a bottom air discharge hole; and a detection mechanism arranged in the housing and in operative coupling with the electromagnetic valve. When the detection mechanism detects moisture entraining gas or air passing through the water accumulation chamber, the electromagnetic valve is activated to allow water to drain through the bottom air discharge hole. As such, operations of automatic detection, accurate draining, and effectively blocking impurities can be all performed and an advantage of maintaining the performance of the pneumatically operated apparatus may also be achieved.