Using the method of ABCD transmission matrix to analyze the positive-branch unstable resonator which consists of multi thin disks, appropriate resonator parameters are got. The self-repeat of resonator is used to calculate oscillation beam size in different disks. The basis of ideal resonator′s threshold gives out the max output energy. Experiment has been carried to verify the analyze, finally we get 10.52 J output laser every single pulse, the magnification is 2.0, the frequency is 25 Hz, the oblique efficiency is 22.75%, the experimental results and the theoretical are totally conformed.