The strapdown inertial navigation system (SINS) is a self-service navigation system widely used in military, aviation, and other fields, and the transfer alignment is one of the key technologies of SINS. The observability analysis of SINS alignment model of large misalignment angle is studied in this paper. Firstly, the nonlinear SINS error model was deduced and the error model was simplified according to the actual engineering situation. Then, based on the observability analysis method of nonlinear theory, the system observable matrix was built using the differential geometry theory, the definition of the system observable degree and the analysis method of the state variables' observable degree were given, and the method was applied to SINS nonlinear transfer alignment model under two different matching modes. Finally, the unscented Kalman filter (UKF) was designed for the SINS large misalignment angle transfer alignment simulation, and the simulation results verified the observability analysis.