The output beam of a slab laser is rectangular with high aspect ratio, and its divergence angles in horizontal and vertical directions are quite different. It contains large phase aberrations which greatly limit the beam qualities of slab lasers. A beam shaping and aberration compensation system is designed for a slab laser with the wavelength of 1064 nm in this paper. This system consists of 3 cylindrical and 1 spherical lenses. Cylindrical lenses are able to change the propagation of light in a particular direction, and the combination of two cylindrical lenses with different focal directions can achieve beam shaping in two dimensions. By changing the distance between the lenses, the rectangular beam can be shaped into square collimated beam, whose size varied in the range from 15 mm×15 mm to 19 mm×19 mm with low phase aberrations, while the size of the input beam is 15 mm×1.5 mm.