A compensation method based on radial preload is presented in order to decrease the surface deformation of a precision mirror caused by gravity. By taking the mirror used in a high numerical aperture (NA) lithographic objective lens as a research object, a force analysis model is established. The influence law of surface deformation affected by the radial preload is analyzed qualitatively under the action of gravity. Surface deformation of the mirror is obtained under the condition of different preloads by the finite element method. The relationship between radial preload and surface deformation is analyzed through data fitting as well as the relationship between the radial preload and the Zernike coefficient. The analysis results show that the radial preload mainly influences the spherical aberration and the trefoil aberration of the mirror surface, and the compensated surface deformations of the mirror first increase and then decrease with the increase of radial preload. The root mean square value of the mirror surface deformation decreases from 2.009 nm to 0.462 nm, which is the optimal compensation result. Moreover, the mirror surface deformation is measured experimentally under the action of gravity and the radial preload, and the experimental results indicate that the mirror surface deformation caused by gravity decreases by a mean of 0.988 nm when the radial preload is about 25 N. The validity of the analysis process and the compensation method are confirmed.