To solve the problems existing in the observable degree analysis method based on the SVD of the observable matrix of linear time-varying systems, such as the inconsistency of state dimensions and the nonuniqueness of singular value reference, an improved method was proposed. First, the relationship between linear time-varying system and piecewise linear constant system (PWCS) was expounded, and the PWCS observability analysis theory was introduced. Under the condition of satisfying the theorem, the complexity of the analytical calculation could be effectively reduced by the stripped observability matrix (SOM) instead of the total observability matrix (TOM). Then, the SVD of SOM and its singular value as well as the corresponding singular vector were extracted by the system. Next, the observation equation of the system was deduced, and the observability index of each state of the system was obtained according to the longitudinal comparison of the observation degree of the same state under different maneuver conditions of the carrier. Finally, the SINS/DVL integrated navigation system was used for simulation verification. Simulation results show that the observability index calculated by this method was consistent with the Kalman filter state estimation error characteristics, which proves that the improved method can predict and accurately describe the state estimation effect. Meanwhile, the system adaptive feedback correction was performed according to the calculated state observability, which could effectively improve the navigation precision.