Abstract: In this paper, frequency domain equivalent model of hydraulic positive displacement pump is presented.Its equivalent parameters（a source flowQs and a source impedance Zs）have been obtained by identification method. With these parameters, a mathematical model of a hydraulic system supplied by parallel double pumps is established. A pressure ripple equationis derived, and a computer program is designed. An aircraft hydraulicsystem is computed with this program. The computed result is in accord with the experimental result. This proves the usefulness of the theoretical equation and the computer program.