Knowledge of borehole heat exchanger efficiency is necessary to optimize the design and performance of ground source heat pump systems. To evaluate the heat transfer performance of the wildly-used vertical U-pipe BHE, a novel one-dimensional numerical model was developed to assess the thermal transfer performance of the BHE from short-term (thermal response test) to long-term (a heating period) for engineering application. The proposed numerical model took into account the internal capacity of the borehole and the thermal resistance between the two legs of U-pipe which are often negligible in traditional one-dimensional numerical models. A thermal response test data from a case at Vorbasse, Denmark and the data from the TRNSYS model were used to validate the feasibility and reliability of the presented model. Then, both a short-term thermal response and a long-term temperature development of the fluid in BHE and surrounding ground were simulated and analyzed based on the model. Additionally, this study did a sensitivity analysis to see what effect the parameters have on the BHE efficiency. Hereby the impact of given assumptions can be able to estimate and the results can serve for the optimum design and control of GSHP systems. (C) 2020 Elsevier Ltd. All rights reserved.