In order to improve the observability and precision of initial alignment of traditional vehicle-mounted Strap-down Inertial Navigation System (SINS), a multi-position alignment method based on the theory of spectral condition number observability analysis is researched.The error model of SINS stationary base alignment is established, the Piece-Wise Constant System (PWCS)theory based on spectral condition number is used to analyze the observability of single-position alignment, optimal dual-position alignment and three-position alignment.The estimating error and alignment precision of inertial devices are contrastively researched, and time distribution of optimal dual-position alignment is discussed. The conclusions are achieved that the optimal dual-position alignment gets higher precision in the same alignment time, and three-position alignment can improve the estimating precision of orientation gyro for the subsequent navigation. The correctness of theoretical analysis is verified by simulation.