In order to optimize beam quality of high power slab lasers, the oscillation of some high-order modes in the cavity was limited by increasing the size of unilateral fundamental mode beam. For semiconductor side-pumped slab lasers, horizontal and vertical thermal focal lengths of the laser under pumping were measured. The equivalent model of thermal lens was established. By using flat-flat cavity as reference, a flat-concave cavity with enlarged dimension of unilateral fundamental modes in both horizontal and vertical directions was designed. It was verified that, for slab lasers with fixed cross-section size of laser medium, expanding the size of one-sided fundamental mode can limit the higher-order mode and optimize the beam quality. A method for further optimizing the performance of slab lasers was proposed. The results show that, when the length of flat-concave cavity is 370mm and the output power is 59.9W, horizontal M2 factor is significantly optimized from 115.6 for flat-flat cavity to 32.9 and vertical M2 factor is significantly optimized from 116.4 to 60.9. This research has practical significance for obtaining high quality output of slab lasers and the related application.