Laser beam shaping system based on deformable mirror has the advantages of control flexibility, good adaptability and high damage threshold. In order to enhance performance of the laser beam shaping system with a deformable mirror as the phase modulation element, a stochastic parallel gradient descent algorithm is proposed to further optimize control voltage, which takes the least square fitting control voltage as the initial value. Laser beam can be transformed to square and circular flat topped beam with different sizes by a 37 element deformable mirror. Numerical simulation results show that, for the far field intensity, both the uniformity of target area and the similarity to the theoretical intensity are improved, after the introduction of optimization algorithm, on the basis of the least square method. So the performance of the laser beam shaping system is also improved.descent algorithm