Predicting the long term ground temperature distribution is essential in the design and application of ground source heat pumps (GSHPs). An improved 3-D line source model is developed to calculate the soil temperature distribution and inlet/outlet fluid temperature to reduce the computation load. Analytical solutions are applied inside the borehole heat exchangers (BHEs) to integrate the ground temperature distribution with the coefficient of performance (COP) of GSHPs. In order to improve the computational stability and speed of 3D numerical modeling, the Alternating Direction Implicit (ADI) method is used to discretize the unsteady energy equations. The convective heat transfer is set as the floor boundary condition to account for the effects of air. This model is validated by experiment data in literature. Results have shown that this model can accurately and quickly predict the long term ground temperature distribution, especially in areas where apparent soil stratification exists. (C) 2017 Elsevier B.V. All rights reserved.