A new predictive model for the coordination number was proposed on the basis of the theory of radial distribution function. It was tested extensively by the Monte Carlo computer simulation data of square-well fluids.This model could predict the properties of square-well fluids with reasonable accuracy and no more parameters were needed. A simple local composition mixing rule independent of density was established and combined with the modified Peng-Robinson equation of state to constract a new vapor liquid equilibrium calculation method. This new method was used to correlate the vapor liquid equilibrium data for nonideal mixtures at low pressure successfully.