In order to improve the uniformity of high-power solid-state laser pump light beam,the basic theory of beam smoothing with imaging microlens array and diffracting microlens array is analyzed.Based on the Fresnel diffraction theory and Fourier transform formula,intensity distribution stochastic solution of microlens array on focal plane has been derived and the homogeneous effects of two kinds of microlens arrays are compared.Meanwhile,the influences of relative aperture of one lenslet and the distance between two microlens arrays on the intensity distribution are researched.The results show that imaging microlens array is of better homogeneous effect and the relative aperture of one lenslet is a key factor to affect the uniformity of light beam.