The composite medium infinite line source model (CMILS) is an attractive model to describe the short time thermal processes of borehole ground heat exchanges. However, this model is very complicated and has a lower computation efficiency which limits its practical applications. In this paper, an equivalent diameter method to the CMILS model is introduced and a new analytical model is developed. The short-term performance of the new model was verified by comparison with three noteworthy analytical solutions and reported experimental data. The impact of the shank spacing of U-tube legs was studied through the new analytical model explicitly. Simulation studies of the equivalent radius were also performed to get the best approach of calculating the equivalent radius with a higher accuracy. (C) 2016 Elsevier B.V. All rights reserved.