Abstract: Turbulent flow over a backward facing step was simulated using a hybrid Reynolds averaged navier stokes (RANS)-large eddy simulation(LES) approach, and the results were compared with experimental data. The hybrid system combines delayed detached-eddy simulation (DDES) with an improved RANS-LES hybrid model aimed at wall modeling in LES (WMLES). It ensures a different response depending on whether the simulation has inflow turbulent content. In the case of with inflow turbulent content, the WMLES branch of the system is active: most of the turbulent is resolved expect the flow near the wall. In the other case, the DDES branch of the system is active,i.e., it gives a pure RANS solution for attached flows and a DES-like solution for massively separated flows. In the system, a scheme which combines the upwind low-diffusion flux-splitting scheme and fourth-order skew-symmetric scheme was used as the convective flux scheme. The spalart-allmaras(S-A)model was applied as a subgrid scale(SGS)model. The definition of the subgrid length-scale was new which depends not only on the grid spacing, but also on the wall distance.