Abstract: The occurrence of earthquake was an extremely complex and highly nonlinear physical process. Based on the X-shaped conjugate shear fracture-seismic composite model proposed by QIAO Jianyong,MA Nianjie and MA Ji,etc, this paper further studied the mechanical mechanism of conjugate shear fracture-seism, and clarified the physical process of conjugate shear fracture-seismic occurrence and its evolution. From the perspective of the surrounding rock stress,failure form around the soft anisotropic body and seismic energy,it was found that the conjugate shear fracture- seism had the law of “Imitation of butterfly survival”. The necessary conditions for occurrence of the conjugate shear fracture-seism were obtained. The research result indicated that there was a butterfly failure zone with extremely unsta- ble behavior in rock mass around the underground soft anisotropic body due to the dramatic change of regional stress environment in which tectonic stress appeared. The expansion of the butterfly-shaped failure zone could eventually form the dominant or recessive X-type conjugate shear fracture. The energy released by each expansion of the butterfly leaf in the butterfly-shaped failure zone could cause an earthquake within a certain range. Therefore,seismicity was the nonlinear dynamic phenomenon of the elastic energy released by the transient expansion of the butterfly-shaped dam- age under dynamic load of triggered event. Conjugate shear fracture was divided into the periods of initiation,growth, upheaval of circular and elliptical failure,butterfly progressive failure and butterfly severe failure under different stress states. According to the strength of Richter magnitude of earthquakes,seismic activities existed weak earthquake peri- od,medium-strong earthquake period and strong earthquake period during the dynamic generation of conjugate shear fracture. The necessary conditions for the occurrence of an earthquake were summarized as follows:the presence of soft anisotropic bodies, the drastic changes of tectonic stress, the strength of those rocks surrounding soft anisotropic bodies,and the stress-triggering condition of an earthquake.