In order to solve the problem of poor output beam quality of slab lasers, we establish a thermal-induced wavefront-distortion model of the lasers transfer along zigzag in slab laser module. Distribution characteristics of thermal-induced wavefront-distortion are studied and a method for reducing thermal-induced wavefront-distortion is proposed. Numerical simulation results show that the position offsets of laser beam propagating through the slab medium twice at the same angle can be optimized to eliminate the thermal-induced wavefront-distortion in thickness direction of slab with the help of reasonable optical path design.