Abstract: The concept of entropy increment ratio (s-) is proposed in this paper, based on a series of direct numerical simulations (DNS) of boundary layer flows on plates at different Mach numbers (Ma=0.7, 2.25, 6). s- represents the dissipation per unit mechanical energy, and is numerically monotonic and independent of Mach number changes, so it can reliably characterize the range of boundary layers. Employing this concept, we reconstruct the length scale of Baldwin-Lomax turbulence model (BL) and bring forward BL-entropy. Flow fields of a backwardfacing step at lowspeeds and a cylinder with conical flare at hypersonic speeds are numerically simulated to evaluate the performance of this new model. The results from the original BL model (BL-origin) and one equation Spalart Allmaras model (SA) are also included to be compared with the available experimental data. The comparison shows that BL-entropy could conquer the essential deficiency of the original model, providing a more physically length scale and smoother eddy viscosity distribution.