The selection and mixture ratio of the mixed refrigerants are important to a multistage auto-cascade refrigeration system. A simulation method was found for the mixed refrigerants selection and ratio calculation by using the commercial software REFPROP8.0. The saturation pressure and evaporation temperature of the auto-cascade refrigeration system were compared to get the optimal mixture ratio of the mixed refrigerants

that is R600a/R23/R14/R740=45/21/19/16. The temperature of freezer was -150℃ at this mixed refrigerants and the mixture ratio. The results show that the research of multistage auto-cascade refrigeration system can be guided by this simulation method and the expected temperature was got.