The geological structure of the Eryuan MS 5． 5 earthquake area is complex，with the two fault groups striking NW and NE． The filed investigation and exploratory trench showed that the Weixi-Qiaohou fault nearby the epicenter is one of the late Quaternary，with its new activities mainly in late Pleistocene． The signs of its activities can be found in Holocene，The movement is mainly of right lateral slipping with some normal faulting． The seismic intensity of this earthquake distribute as an ellipsoid with a NNW direction，and the intensity of meizoseismal area is up to Ⅶ，with a total area of around 279km2，15km in length and 10km in width． The focal mechanism result reveals that the earthquake source is in normal faulting and strike slipping，consistent with the kinematics characteristics of the Weixi-Qiaohou fault，and that the nodal section Ⅱ has a NW direction，basically consistent with the strike of the Weixi-Qiaohou fault． Combining with the distribution features of seismic intensity，the landslide and collapse，and micro earthquake，we believed that the earthquake was generated by the Weixi-Qiaohou fault． The earthquake area is located in the interior of the Hengduan Mountains，northwestern Yunnan． The great terrain relief and the intense weathering of the surface rock are the physical geography situation of the serious damage． The inhabitant is living in simple mud-wood structure or brick-wood structure building with poor seismic performance，and it is the social reason of the serious damage． In order to decrease earthquake losses，we need to increase the infrastructure in the remote rural areas，to improve the living environment in the countryside，and to enhance the seismic performance of the residential building．