Abstract: In order to obtain simple and compact solid-state lasers, a LD end-pumped slab laser with a folded three-pass resonator was used and its thermal lens equivalent cavity model was established. The equivalent cavity stability and cavity mode radius were simulated and analyzed. The new structure and flat cavity structure were studied comparatively based on experiments. The results show that, when the length of the folded three-pass cavity is 170mm, the power output of 1064nm laser is 21W, light-light efficiency is 16.4%, slope efficiency is 25%, and M² factors in horizontal and vertical directions are 10.8 and 2.76 respectively. Under the same conditions, M² factor in horizontal direction is improved from 152.7 of the parallel-plane resonator to 10.8 of the folded three-pass cavity. The horizontal direction of output spot is compressed from 10.8mm of the parallel-flat cavity to 4.1mm of the folded three-pass cavity. Beam output capability of the simple and compact end-pumped laser with a folded three-pass resonator is verified. The study is of practical significance for obtaining intracavity Q-switched and intracavity frequency-doubled 532nm lasers.