Aiming at the accuracy of Strap-down Inertial Navigation System(SINS) the paper presents a method for realizing SINS initial alignment by the assistance of Dopple Velocity Log(DVL).The DVL velocity error was analyzed and its error model and SINS error model were set up.Output revising was used to revise the velocity of DVL in order to provide accurate velocity for SINS and Kalman filter was used to realize precise alignment of SINS.A simulation was made for the SINS initial alignment assisted by DVL under the uniform direct motion state.The simulation results show that this method can improve the convergence speed and alignment accuracy effectively.