The invention provides a muddy water filter automatic slag discharger, belonging to the field of water supply engineering. The slag discharger comprises a reducing machine, a reducing machine rotation shaft, a rotation scraping bar, a long fiber brush, a filter slag-removal plate, a filter net, a slag collecting and discharging channel, a pure water outlet, a pure water chamber, a bracket, a water supply pipe, a muddy water chamber, a slag discharging hole, a slag discharging pipe, a pollution discharging electromagnetic valve, an electromagnetic valve switch signal emitter and the like. After the muddy water containing solid articles flows to the filter net from the inlet of the water supply pipe, the solid articles thereof are intercepted by the filter net, while the residues on the filter net are brushed to the slag collecting and discharging channel by the rotation scraping bar and the long fiber brush and washed away by the discharge water automatically, thus playing the function of filtrating the muddy water and automatically discharging the slag.