The double role piston pump based on the symmetrical gear and crank-link-slider mechanism driven by servo motor is innovated, its working principle and technical characteristics are introduced. In this new type of piston pump, the hydraulic cylinder piston is drived, and the velocity can be adjusted, but the flow pulse of a single pump is undoubtedly great. So two piston pumps are adopted, whose velocity are controled by NC, then the variable stepless timing in wide range can be achieved. If the velocity in this two pumps |v1|+|v2|=C (C is a constant), the flow curve of two pumps will be coupled into a straight line, in order to achieve constant flow output without the need of complex and inefficient throttle controlling loops, and there has not any power machines and mechanical parts in high-speed rotation ,and the noise pollution is very low. According to the varied circumstance, the high and low pressure conversion can be achieved when the angle velocity in this piston pump is controlled by the instructions. It is also very easy to achieve the dynamic and intelligent control.