Medium-deep borehole heat exchanger (MDBHE) has attracted considerable attention because of its ability to acquire large quantities of geothermal energy. Geological conditions in the medium-deep reservoir significantly affect thermal extraction. This study proposes a transient heat transfer model for MDBHE; the accuracy of the model is validated using experimental data. Using this model, the thermal performance of MDBHE under different geological parameters is investigated based on the operation characteristics of MDBHE. Sensitive analysis highlights the importance of the effects of geothermal gradient, thermal conductivity, and heat capacity of the rock soil on thermal extraction load (TEL) and thermal effect radius (TER) of the MDBHE. Results indicate that geothermal gradient affects the TEL mostly and heat capacity has the greatest impact on TER. In regions with low rock-soil heat capacity, borehole spacing should be sufficiently wide to prevent thermal interference between adjacent boreholes. For continuous operations of MDBHE, the annual average TEL declines with operating year and the declining proportion is the largest in the second year for 4%-6%. Besides, the maximum TER increases, with maximal increasing proportion in the second year up to similar to 80%. Under different geological parameters, the declining proportion of TEL and increasing proportion of TER within the same year are almost identical. In addition, the thermal performance of MDBHE under layered subsurface is presented. Layered RTCs and geothermal gradients have a significant effect on the TEL of MDBHE; and layered subsurface with segmented RTCs and rock-soil heat capacities considerably affects the TER. This study will enable the wide promotion of MDBHE application in different regions. (C) 2019 Elsevier Ltd. All rights reserved.