In order to improve the surface precision of a thin primary mirror in a large aperture telescope at different altitude angles, the active correction procedure based on vibration modal calibration was proposed. For a thin primary mirror with the 620 mm in diameter, 18 mm in thickness and the axial munting in 36 points active support, the lateral mounting in 6 tangent points passive support, the free vibration mode of primary mirror was analyzed by finite element method and the first 10 vibration modes of the primary mirror were calibrated. Their RMS values were unified to 1 000 nm, also the calibration forces were calculated. Furthermore,the surface of the primary mirror with different altitude angles was analyzed, the deformations were fitted by the modal vibration mode using least square method, and the corrective forces were calculated. Finally the corrected surface precision and initiative surface precision were compared, and the fitted surface precision and remanent surface precision were analyzed after the second active correction. Corrected results demonstrate that the deformation (RMS) of the primary mirror is corrected from 27.64 nm to 12.95 nm while it is vertically positioned by using the maximum corrective force of 2.23 N, and it is corrected from 7.68 nm to 2.84 nm while horizontally positioned by using the maximum corrective force of 0.59 N. The simulation shows the algorithm using modal vibration to actively correct the primary mirror surface is feasible.