Efficient heat exchange of fluids within artificial thermal reservoir is the fundamental goal for the development of hot dry rock, and it is crucial to reasonably assess the heat extraction ratio of hot dry rock reservoir containing a single fracture. Based on the relationship between fluid output temperature and thermal power generation, the conventional heat extraction ratio is optimized, and the modified heat extraction ratio, i.e., heat extraction power, which considers the effects of both output temperature and power generation, is proposed. A numerical heat transfer model for fluid flowing in a single fracture using a local thermal non-equilibrium model is developed to investigate the effects of reservoir size, fracture aperture and reservoir running time of hot dry rock on the heat extraction power, and the sensitivity hierarchy is classified. The results show that, in the process of reservoir seepage heat transfer, the heat extraction power is mainly controlled by the mass flow rate, and the peak value, i.e., the optimal heat extraction power, appears with the change of the mass flow rate of the fissure flow; when the reservoir size is the same, the fracture aperture has almost no effect on the heat extraction power, the optimal heat extraction power, and the output temperature, etc., but the heat extraction power and the optimal heat extraction power will be declined with the increase of the running time. Based on the modified Morris Method, the sensitivity of reservoir size, fracture aperture and running time to the optimal heat extraction power was analyzed, and it was found that the reservoir size was highly sensitive and positively correlated with the optimal heat extraction power, the running time was generally sensitive and negatively correlated, and the fracture aperture was not sensitive. The research results can provide a reference for the evaluation of heat extraction performance of hot dry rock.