Abstract: To construct the kinematics and inverse kinematics models of the strapdown 3 axis stable tracking system, under the condition of moving vehicle, the method of tracking moving object by 3 axis system was analyzed in detail. Coordinates of the system and their related transformation matrixes were provided first, then the inverse kinematics function was set up with the directional cosine matrix. Three independent joint variables were solved by elements in the matrix. The actual output of the system was obtained by substituting the joint variables into the kinematics function. In the cases of stabilization and autotracking, the simulation outputs are in the same way as the inputs of tracking signals, which verifies the correctness of this mathematical model.