A quasi-steady state simulation was conducted on the cooling process of freeze-drying for pharmaceuticals. It is found that, unlike the system with a pure refrigerant, a peak value of the volumetric refrigerating capacity occurs as the cycle pressure level varies if composition and pressure ratio of mixture refrigerant are specified. After optimization of volumetric refrigerating capacity with constant and varying compositions, further analysis was carried out about the regular effects of mixture refrigerant on refrigerating capacity at varying temperatures, achieving design of mixture refrigerant which can satisfy the requirements of various cooling rates in refrigeration.