Based on thermodynamic and statistical physical analysis, a new mixing equation of state (EOS) for calculating properties of mixed substitutes only with critical parameters of pure substitutes and without experimental data of the mixture was established in this paper. The new method use EOS of pure substitutes to calculate thermal properties of mixed working substitutes. It has a higher degree of accuracy than Peng-Robinson EOS with reckoned interactive coefficient Kij given only critical thermodynamic properties of the working substances. Based on thermodynamic and statistical physical analysis, a new mixing equation of state (EOS) for calculating properties of mixed substitutes only with critical parameters of pure substitutes and without experimental data of the mixture was established in this paper. The new method use EOS of pure substitutes to calculate thermal properties of mixed working substitutes. It has a higher degree of accuracy than Peng-Robinson EOS with reckoned interactive coefficient Kij given only critical thermodynamic properties of the working substances.