In　order　to　simulate　and　analyze　the　soil　temperature　field　of　borehole　groups　in　different　working　condition,　a　four　boreholes　vertical　U-shaped　pipe　heat　transfer　model　outside　the　boreholes　in　cylindrical　coordinate　is　established.　Using　the　Virtual　Heat　Source　and　Green　function　method,　based　on　g-function　and　the　unsteady　three-dimensional　heat　transfer　model,　the　four　boreholes　model　gives　out　a　numerical　simulation　on　the　heat　transfer　process　in　the　case　that　the　soil　is　isotropic　medium　with　MATLAB.　Compared　to　the　experiment　the　the　accuracy　of　the　model　is　verified.　By　the　simulation　results,　intermittent　running　is　beneficial　to　the　recovery　of　soil　temperature　and　small　running　ratio　is　better　than　large　one