The use of slab geometric structure in laser can greatly reduce thermal effects of laser medium, but further analysis of its impact is necessary for optimizing the efficiency of lasers. We analyzed the thermal effects in the partially end-pumped slab laser with a hybrid resonator, using finite element analysis (FEA) method. The calculated results of the thermal focal length match with the measured results. The thermal effect on mode-match was analyzed. The results are helpful for the optimization of laser efficiency and resonant cavity design. In the experiment, an output laser of 41.5 W power is obtained with the input power of 110 W. Optical-optical conversion efficiency is about 38%, and the slope efficiency is 58.8%, with M2x＝1.59 and M2y＝1.55.