Noise reduction in piston pumps has been being the focus research. Many methods have been designed to smooth it. Research shows that noise takes place at the time when pistons enter the discharge port or enter the suction port. In fact, noise is induced by imbalance pressure in piston chamber. In this paper, piston initial volume and instantaneous controlled volume are deduced. Equations describing pressure angle-rate-of-change are also deduced. Keep the pressure increment equal when the pump eccentric distance is maximum and minimum, and then find out the optimal structure parameters. The parameters makes the pressure ripple minimize and makes the lowest noise.