On March 27，2018，the laying hens successively died following the symptoms of fever，diarrhea，hypsokinesis of head and neck in a farm in Shanxi Province. A total of 699 laying hens died from March 27 to March 31，with an attack rate of 86.08%（699/812），which was confirmed to be an outbreak of H7N9 subtype highly pathogenic influenza by national avian influenza reference laboratory. In order to identify all causes and distribution of the outbreak，an investigation was carried out by means of field autopsy，laboratory testing，questionnaire and investigation to find out any possible risk factors and put forward suggestions concerning prevention and control of the disease. It was investigated that the outbreak was directly caused by the fact that hens were not immunized with H7N9 influenza vaccine，and likely caused by the pathogens introduced by personnel，vehicles and wild birds. At last，the outbreak was effectively controlled by a series of emergency responses，and no new case was detected in the county. Therefore，poultry should be immunized with appropriate vaccines against local prevalent strains to establish effective barriers of immunization，and the bio-safety control for personnel，vehicles and wild birds should also be strengthened，which was of great significance for the prevention of H7N9 influenza.