Abstract: This paper solves the coupled heat transfer process inside and outside the borehole wall of a vertical single U-tube ground heat exchanger by using the Mutipole method,analyses the steady-state heat transfer process within the borehole based on the simplified three and four thermal resistance models,parametrically investigates the influence of the pipe layout,thermal properties of grout/soil and other factors on the direct thermal resistance between pipes in the borehole,reveals the mechanism of the negative thermal resistance phenomenon between pipes in the borehole,and some constructive suggestions have been proposed for the industrial application of the simplified thermal resistance model within borehole.