The invention discloses a water hammer air valve and a water hammer gas discharging method during pipeline use. The water hammer air valve comprises a valve body, a bonnet and a protective cover, wherein the bonnet is provided with an opening, the protective cover is disposed on the opening, the valve body is provided with a valve opening for being communicated with a pipeline, a high-speed gas feeding and discharging device which is used for discharging gases stayed in the pipeline during water filling to the outside and shutting down the valve after pipeline water filling is completed is disposed in the inner cavity of the valve body, the output end of the high-speed gas feeding and discharging device is communicated with the outside through the opening of the bonnet, and the valve opening of the valve body is communicated with a bottom gas discharging throttling device for limiting the gas displacement of the pipeline to the inner cavity of the valve body; a trace gas discharging device for discharging mixed, residual and/or precipitated gases in the pipeline to the outside through the opening of the bonnet after the high-speed gas feeding and discharging device shuts down the valve is further disposed in the high-speed gas feeding and discharging device. By the water hammer air valve, water hammer reduction and pipeline safety protection are achieved, blowing blocking during high-speed gas discharging is avoided, and valve closing water hammer is eliminated.