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Geothermal energy is one of the most promising renewable energy. In order to improve the overall utilization degree of medium-and-high temperature geothermal water by flash cycle, a new combined heating and power system is proposed to provide electricity and domestic hot water at the same time. This new system is integrated by single-stage flash cycle and ammonia-water absorption heat pump cycle, of which the heat pump cycle recovers two streams of waste heat of the flash cycle, including the waste heat of the turbine exhaust steam and the geofluid drained from the flasher. The mathematical models of the new system is established at length in this article, and a numerical simulation is conducted to present the preliminary design condition. The simulation results show that the thermal efficiency and exergy efficiency of the system could reach 81.8% and 46.99% respectively under the condition of 170℃ geothermal water. Then a thermodynamic parameter analysis is carried out to explore the impact of five key thermodynamic parameters on the system performance. The results show that an optimal flash pressure happens to make the system exergy efficiency maximal, and there is a similar case for the generator temperature. Within some certain value ranges, a higher rectification column pressure brings about a sharp drop in the exergy efficiency, whereas a higher ammonia concentration of ammonia-strong solution leads to the exergy efficiency rising. Additionally a higher evaporation pressure just has slightly positive effect on the exergy efficiency. Finally an optimization and comparison study is implemented between the proposed system and four flash cycle based combined systems, and the optimization results indicate that the proposed system has a better performance no matter from the viewpoint of the first law or the second law of thermodynamics.