In this paper, we present a novel axial piston water-pump with piston valve, which carries out the flow distribution to drain and sucks water by the interaction of each piston. This novel water-pump could achieve change in-phase between flow rate and the passage areas of the intake and outlet ports. In order to completely achieve the in-phase change, the pressure that is assigned a fixed value in the piston chamber is optimized. On these grounds, the optimum shapes of the intake and outlet ports have been found to be rectangular according to theoretical analysis. The simulation results indicated that the optimum intake and outlet port shapes keeps the pressure in the piston chamber constant, and there is almost no pressure surge except for a very small pressure wave in the piston chamber. Finally, the test of model machine showed that the pressures of the intake and outlet port are very steady, but the leakage is large and the volumetric efficiency is 74.7%.