A general cycle model of a two-stage combined refrigeration system is established and used for analizing the influence of multi-irreversibilities, such as finite rate heat transfer, heat leak between the heat reservoirs and internal dissipation of the working fluid, on the performance of the refrigeration system. The coefficient of performance is taken as an objective function for optimization. The maximum coefficient of performance is calculated, and other corresponding performance parameters, such as the temperatures of the working fluid in the isothermal processes, the optimal distribution of the heat transfer areas and the power input of the refrigeration system, are determined. The results obtained here are more general than those obtained li om a two-stage endoreversible combined refrigeration system and can guide the optimal design and operation of real combined refrigerator systems.