Abstract: This paper presents a method of obtaining distorted wavefront and farfield spots generated by ideal beams from a mirror of laser induced deformations, through the calculating the interferograms of mirrors which are obtained by the experimental method of equivalent power density of pure absorption, and discusses a new idea of measuring farfield spot of high power laser. The results of quantitative analysis indicate that, for pure Si mirror, when the power of pure absorption is 210W and irradiating time is 6s, the maximum thermal deformation is 760nm，and because of deformations the defocusing of nearfield wavefront 320nm， primary coma along x axis and y axis is 130nm and 220nm respectively, the tilt along x axis is up to 120nm, and the beam quality factor is 5.7.