A comprehensive and accurate numerical model is presented based on basic coupled equations to describe multi-Stokes Raman lasing performances in optical fibers. Though much work has been done on this,there are insufficiencies and shortcomings more or less in the literatures after analysis of them. These insufficiencies are also pointed out in this paper. The interactions between forward and backward traveling waves together with interactions between Stokes at different orders are all taken into account in this model. Effective core area for the interaction between multi-Stokes has also been obtained. The numerical solution of the differential system will lead in a natural way to calculate Stokes powers at the output end of the fiber. The proposed model is important for the design of CW cascaded Raman fiber lasers.