A jet stream injecting a supersonic external flow is a common technique for the f1ight control of an aerospace vehic1e. In this paper，MacCormack explicit scheme and Baldwin-Lomax algebraic turbulent model are employed to solve the 2D compressible RANS equations for the numerical simulation of the supersonic external flow Over a rearward facing step with transverse injection. The velocity vector field，pressure contour and Mach contour of the field are given. The step bottom pressure increases by about one time as compared with the case without injection.