A reversing valve for a hydraulic piston pump, including a pilot valve and a main valve. The pilot valve includes a pilot valve seat, a hollow valve core and a pull rod. The main valve comprises an upper valve seat, a lower valve seat and a main valve core. When the hollow valve core is at a upper position, the control flow passage communicates with the spent fluid flow passage, and the main valve core is at a lower position, so the power piston is driven to move downwardly by the power fluids provided by the main valve. When the hollow valve core is at a lower position, the power fluid controls the main valve core to be seated at the upper position, and the power piston is driven to move upwardly by the power fluids provided by the pilot valve.