A method for initial alignment of strapdown inertial navigation system (SINS) is proposed for vehicles under complex swaying condition. First, the coarse alignment (CA) with inertial frame is first designed to address the CA on a sway base, and a FIR( finite impluse response) digital lowpass filter is used to prefiltering the accelerators output. Second, the fine aligment using Kalman filter is deduced with inertial frame. At last, the performance of the algorithm is validated by three properly designed experiments. The results indicate that the method can achive the same precision in the complex swaying condition as in the static base.