When collimator with big diameter and long focal length is used to simulate laser far-field character, the deformation of its lens under gravity cannot be ignored. To analyze the effect, finite element analysis software ANSYS is adopted to simulate a ??400 mm plano-convex lens from a collimator, and a method is presented to analyse wavefront error caused by axial deformation of lens, P-V and RMS values of axial deformations under gravity are computed in different cases, the contours of lens surface are plotted when RMS values of axial deformations is the smallest, the P-V and RMS wavefront error in ??350 mm are also calculated, finally the wavefront error of lenses is estimated in optimal case, and the rationality of collimator design is validated.