Under large pitch angles, in the coarse alignment of the Strapdown Inertial Navigation System(SINS) using dual-vector attitude determination, the accuracy of the attitude angles, especially the azimuth angle, varies greatly under different vector combinations, and must be carefully selected. To address this, two of the most commonly used coarse alignment vector combination error equations are presented, and the main error-affecting factors are analyzed. Considering sensor errors, the accuracy of coarse alignment under full attitude angles is simulated using the Monte Carlo method, and a static coarse alignment experiment was conducted on a turntable at a pitch angle of 80°. In the turntable experiment, with SINS gyro error of about 1°/h and accelerometer error of about 0.1mg , the maximum azimuth angle coarse alignment errors for the first and second combination modes were 20° and 7.5°, respectively, with the second combination mode being about one-third of the first. In the simulation test, with SINS gyro error of about 0.1°/h and accelerometer error of about 1mg , at a pitch angle of 80°, the maximum azimuth angle coarse alignment errors for the first and second combination modes were 11° and 2.2°, respectively, with the former being about five times that of the latter. The results of the simulation and experiment show that the vector combination mode under large pitch angles is the most important factor affecting the accuracy of the coarse alignment of the SINS attitude angles and attitude matrix.