In order to improve the beam quality of output beam in high power laser system, a method to detect defocus by using three spot sizes was presented, the spot sizes at three positions in the axial direction of the beam on the CCD detector was detected using the method, the defocusing amount of the beam can be calculated by the size relationship among the spots, and then closed-loop correction was performed according to the defocusing amount. In the experiment, the defocusing amount of the beam was compensated by adjusting the shaping system, so that the defocusing amount of the beam was corrected from (4.9±0.2) mm to (0.3±0.1) mm. The experimental results show that the method is simple in structure, and can measure strong laser convergence and divergence caused by various factors. The study provides reliable criteria for defocusing compensation, and a method to solve the problem of beam divergence caused by strong laser thermal effect.