On May 14 and 15，2015，large numbers of wild black-necked grebes were found dead in a freshwater lake in Inner Mongolia. Preliminary diagnosis showed that the reason of mass mortality was suspected to be avian influenza（AI）infection. Subsequently，relevant experts organized by Inner Mongolia animal disease prevention and control center rushed to the scene to conduct an epidemiological investigation of emergency，so as to analyze and find out the death reason and evaluate the new risk for local poultry farming because of the death of wild birds. The investigation results showed that the wild black-necked grebes died in this incident with a death rate of 46.2%，and there was no death of other poultry and birds. In addition， case of infection was not found in this lake's surrounding areas of a range of 5 km. Sampling detection for wild birds and poultry in this region was conducted，no pathogen of AIV was detected. However，the qualification rate of post-vaccination antibody against H5 subtype AIV was relative low，and H5N1 subtype highly pathogenic avian influenza（HPAI）viruses were found in all the collected epidemic materials of the dead wild birds by the national avian influenza reference laboratory of China（NAIRL）.Among 7 risk factors of poultry infection by disease transmission from the wild birds，the poultry feeding methods and its vaccination situation were two major factors，possessing a high risk level. According to the investigation，the death was caused by AIV carried by wild black-necked grebes themselves and the epidemic risk of the poultry in this region was low.