In this paper the drift of beam position on target，together with its error distribution, duo to motion of an optical component caused by structural excitations or thermal excitations in laser bay are reported. The contribution to the change of the beam position on target for each optical component (or parameters of laser system) were analyzed with a ray-trace software, in which the drift of focal spot position about target were computed by adding random perturbation to parameters of optical component (by augmenting a perturbation to the matrix elements of corresponding optical component).