An ideal thermodynamic model of a complex heat-work conversion system with two finite heat reservoirs is constructed, which indicates the performance limit and optimization potential of a practical two heat reservoirs system with approximately constant heat capacity. The ideal thermodynamic model is qualitatively and quantitatively classified by analyzing the exergy balance relationship between the heat engine region and the heat pump region. The analytical solutions of thermodynamic performance limit and dimensionless process parameters are solved. The influence of different external work input or output conditions on the thermodynamic performance of the system is emphatically analyzed. The results show that the maximum process work of the parallel flow ideal thermodynamic model is more conducive to realizing the thermal process with a large temperature drop. The ideal thermodynamic model is used to evaluate the ejector large temperature drop cogeneration system. The results show that the system heat exchange perfectibility of thermal output mode, power output mode, and cogeneration mode is 62.70%, 43.44%, and 54.13%, respectively. Affected by the efficiency of the expander and generator, the net power output of the ejector large temperature drop cogeneration system with a total heat exchange of 15 kW is only 0.38 kW, and the thermal efficiency of the system is only 4.57%, which indicates that the thermal performance of the system still has great potential for improvement.