For a ground coupled heat pump system, the variations of ground surface temperature and undisturbed subsurface temperature have pronounced effects on the heat transfer between the ground heat exchanger and surrounding soil. However, how to quantify these effects analytically has long been a tricky issue. In this paper, finite line source model under more complex initial and boundary conditions (FLSCC) is presented and solved based on an existing model, and transient solutions of sub-problems are given, to investigate the effects of ground surface temperature and subsurface temperature variations. To examine the performance of this model, mean borehole surface temperature simulated by FLSCC was compared to that from five conventional models in a case study. Results show that long term behavior of FLSCC is the most close to the reference duct ground heat storage model (DST). The relative mean deviation between the two models at each simulation time step is less than 4.02% from 10 h to 100 years, and the mean value throughout the simulated 100 years is only 1.05%. Also, its short term behavior is close to a validated numerical mode. In addition, the new model is also validated with experimental data from two thermal response tests. Errors between tested and simulated mean fluid temperature calculated by FLSCC are smaller than that of conventional models. Conclusively, the improved model is suitable for dealing with simulation projects at any locations and from any starting point in a year, and also deal with thermal response test data interpretation. It is an effective supplement to finite line source model, and is helpful for fast and reliable practical engineering design. (C) 2019 Published by Elsevier B.V.