Abstract: Many examples show that earthquakes can form conjugate ruptures.The temperature and pressure at the focus are different from that of the shallow part of crust and the ground.The higher temperature and pressure,the larger conjugate rupture angles.Accompanying decrease of depth,temperature and pressure become small and the conjugate rupture angles also become small and same as the conjugate angles of isoseismic lines on the ground and the tensile-shear ruptures are near gound.Based on the method of mathmatical mechanics and the result of rock mechanic experiments,variation of the conjugate ruptures of the Yanyuan-Ninglang earthquake and its limiting principal stresses with the depth of crust is discussed.