Abstract: On the basis of the calculation of phase equilibrium of multicomponent mixture,in this paper,the main equilibrium properties of an optimum gas mixture are calculated using the cubic equation of state,including P-V-T parameters,enthalpy and entropy.The calculation results are shown as the temperature vs.entropy(T-S)diagram.The isobar curves and isenthalpic curves as well as the phase equilibrium curve formulated from the calculation results are presented in this T-S diagram.This diagram makes it convenient and straightforward in the analysis of the thermodynamic process.