Based on the self- reproduction condition of laser wavefront curvature, the influences of disk defocus on laser parameters are calculated. The overlapping efficiency decreases by 9%; the magnification rises to 2.3, and the intra-cavity loss increases to 30% due to a laser beam size mismatch when each disk has a focal length of -100 m in the positive-branch confocal unstable resonator containing four disk with magnification of 1.8. Therefore, the optical conversion efficiency and stability are reduced significantly. Several methods of defocus compensation are compared, it is found that inserting variable- focus lens in resonator is useful in large dynamic range. In the experiment, a lens group used for compensate components with the dynamic range from - 0.016 m- 1 to 0.060 m- 1 is designed carefully and inserted into the resonator. When the pump power is 4840 W , the output power can be improved by 2.33 times compared with that under the non- compensation condition. Under this compensation, the pulse energy of 10 J can be maintained from 1 Hz to 100 Hz.