To solve the problem that the single-frequency ground-based augmentation system cannot meet the requirements of precision approach and landing navigation of aircraft category III, a navigation mode which tightly integrates airborne Differential GPS (DGPS) with Strapdown Inertial Navigation System (SINS) is proposed, and SINS is used to improve the navigation characteristics of the integrated navigation system.The tightly integrated SINS/DGPS filtering model is analyzed, and the state equation and measurement equation of tightly integrated navigation are constructed.The filtering states are estimated and compensated by using the improved Kalman filter.Computer simulation and actual system validation experiments are carried out.The experiment results show that when the satellite signal is not available in SINS/DGPS integrated navigation system, the autonomy of SINS can improve the continuity and availability of the navigation system.Meanwhile, the standard deviation of position error of the integrated navigation system is reduced by more than 50% compared with that of the single airborne DGPS system, and the position guidance accuracy of the navigation system is improved.