In order to improve the navigation accuracy of GPS/INS integrated navigation system under the condition of GPS signal invalidation, tightly-coupled model based on pseudo-range and pseudo-range rate was adopted, virtual observation using the method of virtual satellite was constructed and participated in filtering with real observation. For the problem in the application of nonlinear filtering, the theory of strong tracking filter and RMS strategies were used to improve the CKF. Simulation results verify the feasibility of improved algorithm and the method of virtual satellite, the navigation accuracy can be improved under the condition of GPS signal invalidation and the ability of GPS/INS integrated navigation to adapt to the complex environment are also improved.