The characteristics of Brillouin scattering in silica multimode optial fiber were analyzed based on the ray optics and wave optics theory.A method to determine the range of Brillouin scattering angle was proposed and the expressions of Brillouin shift, Brillouin linewidth, Brillouin scattering spectrum and scattering power of different mode group in step and graded multimode optical fibers are deduced and simulated.The results show that the maximum range of Brillouin scattering angle in step and graded fibers is from twice of the critical incident angle of total reflection to π, and with the increasing of mode group number, Brillouin shift, Brillouin linewidth, normalized peak gain and scattering power of step fiber change more slowly than those of graded fiber, and the above parameters of step fiber decrease nonlinearly in the ranges of 11.084 GHz to 10.932 GHz, 21.760 MHz to 21.168 MHz, 1 to 0.933 and 1.990×10-9 W to 1.857×10-9 W, but the correspending parameters of graded fiber decrease linearly in the ranges of 11.064 GHz to 10.969 GHz, 21.683 MHz to 21.314 MHz, 1 to 0.957 and 2.052×10-9 W to 1.965×10-9 W, respectively.