In order to appreciate the beam quality of 10 12 W high power laser,the spherical surface fitting of reconstructed wavefront was done with least square approximation,and it gave the reference wavefront with no distortion.The diffraction angle of beam of high power laser was calculated with radius of curvature of reference wavefront.The Strehl ratio of the beam can be estimated with the wave aberration between distorted wavefront and reference wavefront.Those quantitative results which appreciated the beam qualities of high power laser showed that the angles of divergence of laser beams were about 10 -4 rad in order of magnitude,and that Strehl ratios were about 10 -17 in order of magnitude,because there were the thermal relaxation processes in the experiments.