Compensation of low order aberrations is essential for high power solid state slab laser. With the increase of output power, the peak-to-valley of wavefront distortion increase to dozens of micrometer. It's difficult to control the wavefront with deformable mirrors which always has limited stroke(<20μm). In this paper, a reflective beam shaping system is designed to shaping the beam spot from rectangular to squarer. The beam shaping system consists of two x-oriented cylindrical mirrors and two y-oriented cylindrical mirrors. Simulations of PID control algorithm for actively compensating of low-order aberrations with reflective beam shaping system are presented. It shows that different combinations of defocus, 0 o astigmatism and 45 o astigmatism, which is the main contributor of beam aberrations in slab laser, can be well compensated by adjustment of distance and rotation angle of mirrors. And the convergence is fast when the control error signal is set to a suitable combination of low order Zernike coefficients. For beam with wave aberrations (PtV=82.6λ, RMS=18.2λ, Z4=23.6, Z5=7.1, Z6=19.6), the adjustment of distance between mirrors is below 100mm, and the rotation angle about z-axis is below 2 degree. The wavefront aberrations are decreased to a low level (PV=0.16λ, RMS=0.04λ) which can be easily corrected later with DM.

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