Abstract: To solve the problem of low heat transfer power caused by thermal penetration in medium-deep single geothermal well circulation（SGWC）systems,a heat transfer enhancement method is proposed in this paper by changing the diameter of inner pipe at the end of the downhole heat exchanger. A numerical model is established by the FLUENT software and a numerical study is carried out with the calculation time of 30 days. The results show that the method of diameter enlargement at the end of the inner pipe can effectively enhance the "interaction" of groundwater and thus more water with high temperature can be pumped from the pumping aquifer to increase the heat transfer power. In this analysis,the downhole heat transfer zone is divided into two parts,which are the conduction zone（CZ）and the pumping and recharging zone（PRZ）. As the blockage ratio increases,the proportion of the aquifer recharge in the pumping water increases,meanwhile,the heat transfer power of the PRZ are gradually increasing in the system. If the blockage ratio increases to 100%,the outlet temperature and heat transfer power can gradually stabilize at 58 ℃ and 995.46 kW respectively,with the heat transfer rate ratio between PRZ and CZ reaching 1.76. Compared to the single geothermal well circulation without enlarged-diameter inner pipes,the heat transfer rates can be increased by 84.71%. Furthermore,under the stable operation,the heat transfer amount flux per meter of SGWC system in the CZ is 154.23-216.89 W/m,which exceeds the highest value of the downhole coaxial deep well heat exchanger,and the ratio of the heat transfer power in the mentioned two systems varies from 3.57 to 6.60 under different operation conditions. The numerical study shows that SGWC system has great advantages.