The thermal cracking of the steam granite is studied under the condition of the non uniform stress state by using the developed convection heating in situ mining simulation test rig and the true three axis press. Under this experimental condition, the granite undergoes brittle fracture, the brittle fracture temperature of the granite is 483℃, and the steam pressure is 10.6MPa. At the same time, under the action of the high temperature and high pressure steam, the main crack of the granite is along the direction normal to the minimum principal stress direction, in addition, due to the thermal cracking, a certain number of randomly distributed secondary cracks in other directions appear. Because of the formation of cracks in different directions, the space crack network connected with each other is of practical significance. The long term experiment study helps to improve the effciency of the geothermal exploitation.