On September 27，2015，sudden mass mortality of chickens in rearing stage occurred for unknown reason in a chicken farm in Zhengzhou city of Henan province. In order to identify the cause and control the disease，an outbreak investigation was carried out. The epidemiological survey was conducted by adopting the following methods，which included asking the feeders，viewing on the spot，animal autopsy，laboratory detection and epidemiological analysis. Based on all of the above detection results，the cause of the disease was confirmed as chicken Coccidia infection. And the outbreak was controlled quickly by mixing feed with florfenicol，disinfecting the poultry houses and cleaning up feces. In view of the analysis，the main risk factors of chicken Coccidia infection were various，including the history of Coccidiosis in this farm，lack of inspection and quarantine when the chickens were introduced，unscientific measures for expelling parasites，as well as not timely cleaning feces. In order to reduce the occurrence of chicken Coccidiosis，corresponding countermeasures were proposed：strengthening quarantine towards the introduced chickens，expelling parasites regularly，cleaning up feces and strengthening bio-safety disposal.