A new method for evaluating the axial stiffness of ball screw at support point was proposed. Based on principle of initial parameter method, the initial parameter matrix equation was established for force and displacement of ball screw. The structural characters, including boundary condition, force balance and displacement compatibility at support point, were used to achieve initial parameter values. As a result, axial force and displacement of every point on ball screw could be obtained. The identification models of axial stiffness were built by analyzing the amplitude changes of ball screw under some load affection employing the knowledge of mechanical vibration and material mechanics. The research results are in good agreement with the results obtained by the available experiments.