To realize initial alignment for a moving state vehicle-carried strapdown inertial navigation system (SINS) and improve the motor ability of vehicle-mounted weapons system, an in-motion alignment method for vehicle-carried SINS aided by odometer was presented, in which the velocity integration formula is derived in body frames and the initial alignment has been transformed into a optimization problem of determining the initial attitude by using the chain rule. Then the initial attitude matrix is solved by using QUEST method and the detailed discretization recursive model of algorithm is given, the auto in-motion alignment is achieved before the navigation start. The simulation results show that the method can accomplish the in-motion alignment quickly and accurately without any prior attitude information.