To improve the accuracy of the integrated navigation system INS (Inertial Navigation System)/CNS (Celestial Navigation System)/GNSS (Global Navigation Satellite Systems) integrated navigation system for long-time working, this paper establishes the integrated navigation system state equation and the observation equation based on quaternion errors, and analyzes the observability and observable degree of the system states by applying Piece-Wise Constant System (PWCS) theory for different input conditions. Based on the analysis of the observability and observable degree, using the measurement information of attitude and position vectors from star sensor and GNSS, this paper proposes a method of on-line calibration for the error parameters of gyros, accelerometers and star sensor. The observability analysis and simulation results show that this method can precisely estimate the scale factor, zero, and installation error of gyros, accelerators and star sensor. This method can improve the long-term working accuracy of INS/CNS/GNSS integrated navigation system and has an important engineering application value.