In some areas of accuracy is not very high, the low accuracy of SINS like MEMS have wide applications for its low cost, but MEMS does not apply to the conventional initial alignment because they can estimate nothing but posture. A kind of initial alignment based on EKF is studied in this paper, and the multiplicative extended Kalman Filter (MEKF) is used for aligning. The estimation of inertial device error modeling is realized as the posture alignment. The amount of calculation is reduced significantly while the initial alignment is realized.