Objective To investigate an acute gastroenteritis outbreak and discuss the methodology of etiology screening. Methods Cases were identified according to the definition. Descriptive epidemiological approaches and case-control study designs were employed in the analysis. All the samples were tested for common pathogens by isolation. Results A total of 43 cases were found. Epidemiological curve suggested that it was a point source outbreak and the major symptoms were diarrhea (100.0%,43/43),abdominal pains (93.0%,40/43),nauseous (32.6%,14/43) and so on. Results From the case-control study revealed that the roast duck served at lunch on September 12 was suspectable. A total of 36 samples, including rectal swabs from patients, hand swabs from kitchen workers, leftover food and environmental swabs, were collected.Five samples (collected from 13 patients) were positive. Conclusion This was a suspected foodborne outbreak of Clostridium perfringens. The possible cause was that the roast duck was contaminated by Clostridium perfringens when stored in the external environment at room temperature for about 5-6 h and was directly ate by patients without reheating. It is necessary to strengthen the supervision and administration of chefs, and improve health consciousness of the public to prevent such case. Investigators should abandon the “lab-dependent” way and strengthen the summary of symptoms-based epidemiology related to a variety of agents so as to decreases the rate of erroneous diagnosis and misdiagnosis dued to the unfamiliar of symptoms of various foodborne diseases.