A modified partially averaged Navier–Stokes model (MPANS) is proposed by treating the standard k–ε model as the parent model and formulating the unresolved-to-total kinetic energy ratio fk as a function of the local grid size and turbulence length scale. Flows over a backward facing step are used to evaluate the performance of MPANS mode. Computations of the standard k–ε model, the constant fk partially averaged Navier–Stokes (PANS) models (fk = 0.6, 0.7), and the two-stage PANS model are carried out for comparisons. Based on the detailed analyses of calculated results and experimental data, the MPANS model performs better to predict the reattachment length together with the corner vortex and provides overall improved statistics of skin frictions, pressures, velocity profiles, and Reynolds stresses, demonstrating its promising applications in industrial turbomachines that often encounter with flow separations.