A method of the core area replaced by the optimum Gaussian mode area was proposed when calculating the effective stimulated Brillouin scattering gain coefficient. Then, the effect of steady-state Stimulated Brillouin Scattering (SBS) in a long multimode gain fiber was studied based on the coupled differential equations on beam intensity. The results show that the residual pumping power becomes saturated, and the Stokes power increases linearly with the launching power for whether the first-order or second-order SBS process. This study also shows that the second-order Stokes power is relatively weaker, and the effective length of the fiber decreases gradually with the increase of the launching power.