On the base of analysis on running condition and failure of hydraulic system,a model was built to simulate hydraulic system,and a virtual system which could reproduce the running data of hydraulic system repeatedly was developed,the virtual system could be used for the study on running state monitoring and failure diagnosis system of hydraulic system.In this paper,the techniques for the software development are discussed,such as mathematical modeling,object-oriented analysis,function interpolation,and solving the nonlinear equations,the virtual system is explained also.