In this paper,a deformable mirror is used to simulate mirror thermal distortion induced by laser,which deforms by different drive voltages applied to it.The deformation is measured by a Hartmann-Shack sensor by the method of normal incidence and non-normal incidence respectively.In contrast to the true peak to valley value（PV value ）of wavefront aberration which is measured by normal incidence method,the PV value of wavefront aberration measured in non-normal incidence method is less.The relative deviation between them increases with the increasing of the deformation,and closely relates with the incident angle.With the increasing of the incident angle,the relative deviation increases significantly.In order to correct the deviation,the PV data which is measured by non-normal incidence method divided by the cosine of the incident angle are analyzed.It is found to match the true PV value well.The maximum relative deviation between them is less than 2%.