Abstract：The research on rheological property，rheological mechanism of surrounding rock of granite under high temperature and high pressure is of great importance to extracting heat from hot dry rock(HDR) and stability of drilling borehole wall. Utilizing the 20 MN servo-controlled triaxial rock testing machine with high temperature and high pressure developed by the authors，the intensive research on the rheological property of large size granite with borehole samples of f 200 mm×400 mm under high temperature and triaxial hydrostatic stress in depth is carried out. The research results can be found as follows. (1) The granite is a kind of brittle and hard rock，it is constituted with multi-crystal. Surrounding rock in granite shows of steady creep state，main with hydrostatic stress below than 5 000 m in depth and temperature below 600 ℃. However，it appears as unstable creep state when hydrostatic stress is over 5 000 m in depth and temperature above 600 ℃. (2) The creep of surrounding rock of granite has a creep threshold of stress and creep temperature threshold. (3) The main reason of surrounding rock possessing threshold of stress and temperature is the dislocation and cracking of grain and cement of grain under thermo-mechanical coupling. (4) At different temperatures and hydrostatic stresses in depths，the critical hydrostatic stress of rheodestruction of surrounding rock is 5 000–6 000 m；and the critical temperature of rheodestruction is 500 ℃–600 ℃. The failure modes commonly are compression fracture，transpression or both of them. At the same time，the parameters of steady creep state rates of surrounding rock within 600 ℃ and hydrostatic of stress of 6 000 m are obtained；thus it can provide significant mechanical parameter for the stability of HDR borehole wall.
