The two-position alignment can increase the precision of SINS initial alignment due to two reasons that are the compensation of inertial devices constant errors and the improvement of system observability. Firstly in order to study the error compensation principle of single-axis rotary SINS, the system error model is established, and the equivalent constant errors of inertial devices is introduced. Then the optimal twoposition alignment scheme is deduced from the view of the equivalent gyro constant drift. A conclusion that two-position alignment can restrain horizontal equivalent gyro constant drift and decrease azimuth error is reached by substituting into the error propagation equations. Finally, the Kalman filter model is established and the method of two-position alignment to improve system obserbability is verified and the relevant experiment is carried out. The results show that the proposed scheme can increase the alignment precision to 36.441 4″(1σ).