The present invention relates to an "improved piston valve for diaphragm pump" features that each of water discharge base and water discharge base has a top camber concave surface with orientating hole center and orientating hole center as lowest point respectively; After anchoring anti-backflow plastic gasket into water discharge base, a gap is formed between bottom surface of anti-backflow plastic gasket and top camber concave surface of water discharge base; Likewise, after anchoring each piston slice into each corresponding water inlet port, a gap is formed between bottom surface of piston slice and top camber concave surface of water inlet port; By functions of foregoing gaps, the overall compressing efficiency is substantially enhanced because both sucking forces of anti-backflow plastic gasket and piston slice are further increased by reciprocal actions of piston pusher.