A reflective beam shaper is designed for the purpose of compensation of the low-order aberrations in high-power slab lasers. A proportional-integral-derivative (PID) control algorithm is used to control the optical parameters of a beam shaper for active compensation of low-order aberrations. Simulations of the PID algorithm show that different combinations of defocus, 0-deg astigmatism, and 45-deg astigmatism, which are the main contributors of beam aberrations in slab lasers, can be well compensated by variation of distance and rotation angle of mirrors. For a beam with large wave aberrations [peak-to-valley (PtV)=87.7λ , root-mean-square (RMS)=19λ ], the adjustment of distance between mirrors is less than 100 mm, and the rotation angle about the z-axis is <3.2 deg , and the wavefront distortion is reduced to a level (PtV=0.50λ , RMS=0.09λ ) that can be further corrected with one deformable mirror. The effectiveness and performance of low-order aberration compensation with the reflective beam shaper are also verified by experiments.