Abstract: The observable degrees of the system states are the key indexes to check the convergence accuracy and velocity of designed Kalman filter. First, the observability analysis method of piece-wise constant system(PWCS)and the observable degree analysis method of the system states based on the singular value decomposition are described respectively in this paper. And then the integrated GPS/SINS navigation system can be demonstrated to meet the law of PWCS analysis method. In order to analyze the observable degrees of linear varying-time system states along a whole trajectory, the modified analysis method will be presented further. Finally, its rationality and feasibility can be shown by a simulating example.