Two styles of inner bypass structure, the annulus style and gear style, were numerically studied. The fuel was HTPB while the overall reaction process and Eddy-Dissipation Model were adapted. It was concluded that the combustion in the dump afterburning chamber of annulus style was still controlled by diffuse. The gear style could induce couples of symmetrical vortexs in the head of dump afterburning chamber, which could enhance the mixing effect of unburned fuel and bypass air, and the total pressure loss was approximately approaching the annulus style, thus the combustion performance of gear style performs better.