Through the approximate three- wave- coupling equations, based on Langevin noise model of the stimulated Brillouin scattering, the stimulated Brillouin scattering process in optical fiber is numerically simulated using the finite difference time domain method. As for the single-mode fiber of 10 km long and with the refractive index of 1.5132, the characteristics of the optical field and the acoustic field of the temporal and spatial variation and the power variation of scattering light within 50 ms are analyzed. In the numerical simulation, under the function of pump beam and with Stokes light as the incident vibration light into the other end of the optical fiber, it is found that whether the polarized light exists or not will affect the amplitude of the temporal and spatial variation of the pump light, scattering light field and acoustic field, and it is obtained that the scattering light power tends toward saturation with time when the Stokes light exists and the scattering light power shows nearly linear variation with time when no Stokes light exists.