Abstract: The utilization of downhole coaxial heat exchanger system（DCHES）is an effective means to develop geothermal energy in rock formation. However,there is currently a lack of design methods to optimize its operating flow rate. Based on the basic principles of flow and heat transfer,a mathematical model of DCHES is established and verified by the field test data. By coupling the availability of heat extraction power,an optimal flow rate design method of DCHES was established. The field application was carried out in Xi’an area as an example. The results show that the availability of the heat extraction power first increases and then decreases with the increase of the flow rate;the flow rate corresponding to the maximum extraction heat power is the optimal flow rate of DCHES. In the operation of DCHES,this method can be used to continuously adjust the operating flow to obtain the maximum exergy power.