Abstract: A theoretical model for the new type phaseconjugator using combined fiber, which is composed of two fibers with different core diameters connected by an internal taper, is established. By numerical calculation the whole SBS process in combined fiber has been illustrated clearly and its key characters have been discussed. The results show that the combined fiber can greatly decrease SBS threshold and improve reflectivity while behaves a much larger dynamic range. Furthermore, with optimum dimensions, the combined fiber can suppress the power modulation of Stokes wave and show a potentiality to considerably compress laser pulse.