The invention discloses an apparatus for automatically discharging wash water in a filling pipeline. The apparatus comprises a monitoring pipe segment, a discharging solenoid valve, a filling solenoid valve and a PLC program controller, wherein the filling solenoid valve and the discharge solenoid valve are arranged at the tail end of the monitoring pipe segment, the monitoring pipe segment is vertically arranged, the monitoring pipe segment is provided with a differential pressure monitoring apparatus, the differential pressure monitoring apparatus, the filling solenoid valve and the discharging solenoid valve are connected with the PLC program controller through a signal line, and two pressure sensors can also be replaced with one differential pressure sensor. Further disclosed is a method for automatically discharging wash water in a filling pipeline. According to the method, whether a material is slurry or the wash water is determined by detecting a material density value at an outlet of the monitoring pipe segment through the PLC program controller, and accordingly, closed states of the discharging solenoid valve and the filling solenoid valve are controlled. According to the invention, whether the material coming out of the filling pipeline is the slurry or the pipe wash water can be automatically determined, then a discharging position of the material is automatically selected, the reliability is quite high, unattended operation can be realized, the structure is simple, and the cost is quite low.