The classical error model of SINS with large azimuth misalignment angle can not exactly express the error transmit characteristics. Since the rotation vector can describe the attitude of SINS, a new model was derived based on quaternions differential equation. The expressions of the new model were also given as euler angle differential equation. Test based on UKF on stationary base shows the new model can expand the range of azimuth misalignment angle from within 10°to within 25° and can get more superior alignment precision for initial alignment with large azimuth misalignment angle. The test result shows that the new model is more accurate.