Abstract: In case of the initial alignment in the strapdown inertial navigation system (SINS) with a large azimuth misalignment, a nonlinear filtering method based on MPF-CKF is proposed in this paper.MPFCKF takes part of the inertial errors as model errors, reduces system dimensions, which not only improves the accuracy of initial alignment, but also overcomes the limitation that model errors are assumed to be Gaussian white noise.Filter simulation shows that MPF-CKF can improve the estimation accuracy and convergence speed in the initial alignment with a large azimuth misalignment in the SINS.