Aiming at the problem of reduced alignment accuracy caused by sloshing interference and constant error of inertial devices during the in-motion alignment process of a vehicle-mounted strapdown inertial navigation system (SINS), an anti-interference moving alignment method for the vehicle-mounted laser SINS based on rotation modulation is proposed.Aiming at the sloshing interference during the alignment process, the attitude real-time update method based on the inertial system is used to track attitude changes so as to overcome the interference of angular sloshing, and the integral for the force equation is conducted to reduce the line vibration interference. At the same time, the attitude at the starting time is obtained by combining the optimal estimation of the attitude. In the process of updating the direction cosine matrix, the equivalent rotation vector algorithm of "monad sample+previous cycle" is adopted to reduce the influence of the non-exchangeable error on the accuracy of attitude calculation. Then, the single-axis continuous rotation modulation method is used to realize the self-compensation of the constant error of the inertial devices. Simulation results show that the autonomous initial alignment method can overcome interference during the alignment process, eliminate the influence of the constant error of the inertial devices, and improve the alignment accuracy.