Based on the coupled wave equation of Stimulated Brillouin Scattering (SBS), the models of Brillouin frequency shift, the pump wave, Stokes wave and the acoustic field in multimode fiber were built. Analysis and comparison of the Brillouin sepectrum frequency, peak gain, linewidth and SBS threshold between the multimode and single mode fiber were conducted. And then the influences of optical fiber length, attenuation coefficient and numerical aperture on the SBS threshold were discussed. The results show that, multiple modes of the Brillouin scattering superposition lead to spectrum broadening, linewidth increase, peak gain decrease and frequency shift reduce. The SBS threshold of the graded index multimode optical fiber is larger than 80 mW, which is far larger than the single mode fiber. When the core diameter is same, the graded index multimode fiber is more susceptible to takeplace the SBS effect, whose numerical aperture has more influences on SBS threshold than the step index multimode fiber. The reported measurement results validate the theoretical analysis. The studies in this paper laid a theoretical foundation for the distributed Brillouin scattering sensing system based on multimode fiber.