A novel method is presented to reconstruct the wavefront with phase distortion in STRALIGHT II 1012 W high power laser apparatus by means of computer image processing. In experiments, the image processing of optical spots fired on photograph by high power laser beams through a plate with hole array. The iteration formula group was deduced to apply in the experiments. Each center of intensitv of optical spots and relative shift were measured, and phase wavefront of high power laser was reconstructed in iteration. Computer software made the focusing procedure of small optical beams in hole array instead of which in sub-lens array of normal Hartmann-Shack sensor. The method can be used to measure directly the high power laser beam with large diameter. The principle and the formula of the reconstruction are described. The method has been used in the experiments of the high power laser apparatus STARLIGHT ⅠⅠ. The maximum diameter measured is 200 mm,and the resolving power of wavefront reconstruction is about λ/5.