Growth rate and gain of stimulated Brillouin scattering (SBS) considering the reduced Landau damping due to particle trapping has been proposed to predict the growth and average level of SBS reflectivity. Due to particle trapping, the reduced Landau damping has been taken used of to calculate the gain of SBS, which will make the simulation data of SBS average reflectivity be consistent to the Tang model better. Other nonlinear effects such as harmonic generation and pump depletion have been also considered in the later evolution of SBS. This work may solve the pending questions in laser-plasma interaction and have wide applications in parametric instabilities.