From April 18 to May 2，2020，a large number of shelducks purchased from other regions by a farm continuously suffered from depression，anorexia，unwillingness to walk，head shaking and other symptoms，and died each day. In order to find out any possible etiological factors，a field investigation was carried out to investigate the status of breeding and management in the farm，morbidity and mortality，treatment and other factors，followed by statistical analysis on the number of deaths. Blood，tissues and cloaca swabs were collected for laboratory test to analyze the distributions of time，regions and populations of the outbreak through descriptive research. Based on the results of investigation and laboratory test，the outbreak was confirmed to be an infectious serositis in ducks，caused by Pasteurella anatipestipestii due to stress reaction and reduced resistance occurred in the ducks with poor management after their long-distance transportation；then the outbreak was spreading without timely control because of delay removal of dead ducks，unscientific dosage and method of administration. Considering that ducklings had a poor ability to adapt to their surrounding environment，for the purpose of reducing any occurrence risk of such diseases，during the process of outsourcing，enough nutrition for them should be ensured，disease resistance should be improved to reduce any possible stress reaction，and vaccination should be timely implemented when they were imported.