We numerically and experimentally study a multiwavelength fiber laser (MWFL) employing a nonlinear Brillouin optical loop mirror (NBOLM). Taking into account the impact of stimulated Brillouin scattering (SBS) effect on nonlinear polarization evolution, we present the power transmission equation of Stokes lines from the NBOLM. Thereafter, we combine the power transmission equation, coupled wave equations of SBS process in NBOLM, rate and power propagation equations in the erbium-doped fiber (EDF) to build up a model for the MWFL. Using this model, we can explain the impacts of EDF pump power, input polarization state and quarter-wave-plate angle on the number and amplitude flatness of output Stokes lines. Furthermore, the results from numerical calculations are verified by the experimental measurements.