The coaxial casing heat exchanger is one of the technologies to realize high efficient heat extraction in the middle and deep geothermal energy wells, and its heat transfer capacity is crucial to the sustainable development of geothermal system. Taking Guanzhong area as an example, the heat transfer model of coaxial casing heat exchanger in middle-deep geothermal wells is established considering the formation inhomogeneity, and the effects of injection temperature, injection flow rate, buried depth and other factors on its heat extraction performance are studied. The influences of injection temperature and injection flow rate on thermal reservoir temperature recovery under intermittent production condition are discussed. The results show that, the outlet fluid temperature decreases with the increasing of injection flow rate, but the heat extraction power of the whole system increases. Increasing the injection temperature can improve the outlet fluid temperature, but the system heat extraction power decreases greatly. With the increasing of formation depth, the temperature of outlet fluid and heat extraction power increase gradually. Reducing the inner pipe diameter and enlarging the outer pipe diameter can effectively improve the temperature of outlet fluid and heat extraction power. The temperature recovery ability of the thermal reservoir increases with the injection flow and injection temperature. Reducing the thermal conductivity of inner pipe or choosing double-layer inner pipe structure can reduce the fluid temperature loss along the inner tube and increase the outlet fluid temperature.