A universal cycle model of an n-stage combined heat pump system, which includes the irreversibility of finite-rate heat transfer across finite temperature differences, the heat leak loss between the external heat reservoirs, and the irreversibilities inside the working fluid, is established and used to investigate the influence of these irreversibilities on the performance of the system. The coefficient of performance is taken as an objective function for optimization. A general optimum relation is derived for a given specific heating load and a given total heat-transfer area of heat exchangers. The general performance characteristic curves of the system are obtained. The maximum coefficient of performance with non-zero specific heating load is determined. Some key design variables, such as the specific heating load, the temperature ratios of the working fluid in the isothermal processes, the distribution of the heat-transfer areas of heat exchangers, and the total power input of the system, are optimized. The optimal performance of an arbitrary-stage irreversible, endoreversible, and reversible combined heat pump system can be directly derived from the results in this paper.