Complicated flow field of shock/boundary layer interaction in the supersonic diffuser is simulated by solving the Reynolds-averaged Navier-stokes equations in 2D,axisymmetric and 3D cases. The length of shock train, the distance of each shocks in it and the level of pressure recovered are compared with each case. Also, the oscillation phenomenon of the shock train is captured through Runge-Kutta method when the freestream mach number is 3 and the results are compared with experimental data.