Ground　heat　exchanger　is　an　important　component　of　the　Ground　Coupled　Heat　Pump　(GCHP)　system　and　a　single　U-tube　is　the　most　common　type　of　ground　heat　exchanger.　In　this　paper　a　full-scale　3D　numerical　model　of　a　single　U-tube　ground　heat　exchanger　was　built.　The　effectiveness　of　the　model　was　verified　by　comparing　the　computed　results　with　the　field　test　data.　The　heat　extraction　rate　of　a　single　U-tube　of　a　GCHP　operating　in　heating　mode　was　numerically　computed,　for　both　continuous　and　cyclic　operation.　The　time　variation　of　the　heat　extraction　rate　was　derived.　This　paper　also　studied　how　the　influence　radius　of　a　single　U-tube　changes　with　time.　The　results　may　provide　guidance　in　the　design　of　GCHP　systems.