The complex inner flow-field of secondary gas injection for Solid Rocket Motor was numerically investigated by using three-dimensional average Favre equations and the turbulent models. Three-order spatial scheme and Implicit point Jacobi algorithm were used to solve these equations. It gained the structures of shockwave series of secondary gas injection vector nozzle and complex main/secondary flow image by numerical simulation. The secondary flow field includes complex vortices and shockwave series. It also has interaction by the boundary layer and shockwave, free shearing layer, shockwave, rarefaction wave and heavy gauge separation. The numerical simulation also indicates that secondary hot gas injection results in high temperature on the surface wall round the injector hole. Corresponding thermal protections should be adopted.