The Closed Loop Geothermal System (CLGS) is a novel method for exploiting geothermal energy. However, to the best of our knowledge, few studies have investigated the heat production performance of CLGS, which is particularly important for its field application. Hence, in this paper, an unsteady-state heat transfer model for CLGS is presented that considers variation of convection heat transfer coefficient. The finite difference method is employed to solve the mathematical model. The proposed model is verified with an analytical solution. Subsequently, the temperature field of CLGS is analysed, and influences of key parameters on CLGS performance are studied. The results indicate that the production process of CLGS can be divided into three sections: decreasing region, transition region and stable production region. The flow rate, inlet temperature and horizontal section length have significant influences on CLGS performance. To obtain the higher thermal power and appropriate outlet temperature, there is a critical value for flow rate. Additionally, the longer horizontal section length and larger temperature difference between the working fluid and formation are beneficial for improving CLGS performance. Finally, small wellbores should be drilled for CLGS. The main findings of this work can offer guidance for the design and optimization of CLGS. (C) 2017 Elsevier Ltd. All rights reserved.