When high-power solid laser working in high repetition rate, the laser output wave-front changes due to the thermal distortion, which is caused by the gain medium under the heat deposition. Therefore, the coherent modulation imaging technology is used to realize the measurement of the laser output wave-front from one frame of diffraction intensity recorded. The phases of thermal distortion of the optical element are obtained when the amplifier working in 1, 5, 7 Hz frequency. Experimental results show that thermal distortion effect is significantly increased, the heat is concentrated toward the center region and the phase change increases as the working frequency increases.