The relationship between the ground temperature field and the precipitation field within one year before the occurrence of strong earthquake（MS≧6.9,14 times in total） in western China over the past 30 years are analyzed.It is found that： ①A large area with abnormal high soil temperature at 3.2 m are identified one year before the occurrence of earthquake accompanied by less precipitation in this area.The abnormal mentioned could last for 2 to 6 months with a maximum dry area of 2-4×106 km2.② The fluctuation in both the soil temperature and precipitation fields with the wavelength less than 1 000 km are appeared 0.5-1 year before earthquake occurrence,namely,the so called ＂ultra low frequency stress wave＂.The wave is responsible for break of the curst.③ The stress wave weakens with a shorter wavelength and lower intensity,as well as a sharp turning of the propagation direction by nearly 90°.The above three stages could be called ＂the three stages for earthquake gestation＂.A hypothesis is proposed on the mechanism of strong earthquake based on the above mentioned facts as follows： There is a kind of unstable uplifting Benard convection in outer layer of the earth core with a horizontal scale of 106 km^2.The uplifting action push up the crust and the mantle.When arrives at the boundary layer between the earth core and the mantle.Following results are expected consecutively： The higher soil temperature field and the heat conductivity decreasing cause less rainfall.With the stress in the crust keeping increase a kind of unstable wave（strain wave） appears.The weakening of the stress wave happens right before the occurrence of the strong earthquake.