For the requirement of high-precision rapid alignment of Strapdown Inertial Navigation System (SINS), a fast initial alignment method with the constraint of constant attitude angle is proposed on the basis of traditional position-observation filtering method.Based on the fact that the equivalent rotation vector of the initial body coordinate system relative to the body coordinate system remain constant or only changes a little under static conditions, the status and measurement models are established, and a closed-loop Kalman filter is applied to implement the fine alignment.Simulation and experiment results show that, the new method can accomplish the alignment with higher precision compared with the traditional methods, and the alignment accuracy of SINS is improved by 43%.In addition, the new method can be easily applied in engineering with fine effects.