The JWL equation of state is used worldwide to describe the pressure-volume-energy behavior of a detonating explosive. Obtaining the JWL constants needs experiments and iterative calculation, and so is expensive in time and economy. The JWL constants found in the literature is only suit for sample of a certain density, while it is simple to have slight density differences in application. In this paper an engineering method to calculating JWL constants while the density of explosive changed is proposed. An example is given out and it is shown good agreement between the experiment results and numerical simulation.