The theoretical and experimental studies on thermal effect of LD edge-pumped slab lasers were carried out. Based on a real edge-pumped slab crystal (2.5 mm×14 mm×40 mm), the three-dimensional (3D) thermal model was built up. Then the temperature distribution and stress pattern of the slab were calculated by finite element analysis. The thermal distortion of slab crystal under 120 W pumping power was obtained, and the thermal focal length for different pumping powers was estimated. The LD edge-pumped slab laser was designed and packaged. The thermal focal length was studied experimentally, and its relationship with pumping power was dicussed. Experimental results agree with theoretical analysis.