In order to solve the problem of multi-borehole ground thermal resistance calculation concerning thermal interference in the national standard, a numerical method for vertical ground heat exchanger is presented in this paper. And ground thermal resistance distributions of different borehole configurations and different separation space between two neighboring boreholes were studied through a case study. The results show that for regular rectangular-array borehole configuration, the ground thermal resistance in the central position is largest, also, the more the configuration reaches a square, the smaller the separation space is, the larger the total subsurface thermal resistance is; while for irregular rectangular-array borehole configuration, the position where largest subsurface thermal resistance is located will exist at a new negligible offset. The research output can provide theoretical reference for the design of vertical ground heat exchanger.