It is important to understand the reservoir rationally for the utilization of geothermal resources. Due to tectonic action or pressure drop in the production stage, large-scale natural fractures often occur near geothermal wells, and there are varying conductivity characteristics inside the fractures. It is difficult to obtain accurate reservoir evaluation results by conventional well test analysis methods. Therefore, based on the theory of source function and potential superposition, considering the variable conductivity of large-scale natural fractures, the dual-porosity medium model is used to characterize small-scale fractures, and the well test model in natural fracture reservoirs is established. The bottom hole pressure calculation is realized by coupling the wellbore, fracture system and reservoir system. The typical well test curves were drawn and the stages were divided. The variation law of well test curves under different types of variable conductivity is analyzed. The correctness of the model is verified by comparing with the commercial software numerical well test method. Finally, the model is used to explain the example. The curve matching degree is high and the interpretation parameters are reasonable, which provides a theoretical basis for reservoir parameter evaluation of geothermal wells in natural fractured reservoirs.