Abstract: For the first time using an optical observation instrument of borehole deformation based on the theory of optics, the intensive studies on deformation laws and stability conditions of borehole within 600℃ and under triaxial hydrostatic stresses at a depth of 6000 m are carried out. The research shows: (1) Under high temperatures and stresses, drilling deformation in granite is different according to the temperatures and stresses at different depths, and it may divide into 3 stages: viscoelastic stage, viscoelastic-plastic stage and destroying stage. (2) Under constant temperatures and stresses, drilling deformation in the granite has obvious time effect. With the time, the wall displacement of borehole gradually increases, the aperture decreases, after the drilling deformation achieves the ration and tends to be stable, the borehole does not have the destruction. (3) Temperatures and loading stresses have very tremendous influence on the drilling deformation. If the loading stresses on the rock mass are kept invariable and the temperature increases, the drilling deformation would gradually increases and the diameter would decrease; if the temperature is kept invariable, the larger the loading stress level, the larger the wall deformation of drilling and the smaller the inner diameter of drilling.