According to the power matching of power mechanism，one form of definition of load flow and load pressure is given. On this basis, the mathematical model is applied to both asymmetrical cylinder and symmetrical cylinder controlled by servovalve. A unified mathematical model is presented to solve the asymmetrical of cylinder. And this theory has been testified by simulation. Then the model of position control system of a certain testing platform is built on this method, and the variety effects of system dynamic performance under different parameters and power source are analyzed．Thereby a theory foundation for the optimal design and control policy of the system is provided.