In order to improve the robustness and precision of initial alignment of strapdown inertial navigation system (SINS) for land vehicles with swinging base, an initial  alignment method composed of inertial coordination multi-vector coarse alignment and optimal estimation fine alignment with rotation modulation is researched. Theory of inertial coordination alignment is stated, and difference between double-vector alignment and multi-vector alignment is analyzed. The model of optimal estimation fine alignment is established, and error suppression theory of SINS alignment with rotation modulation is researched. It is verified by simulation experiments that inertial coordination alignment can overcome  effect of swing, but is influenced by vibration. Multi-vector alignment uses more information of measured vectors and gets more precise alignment results than double-vectors alignment. Rotation modulation inhibits error of horizontal inertial devices and achieves better precision than single position alignment.