Objective: To identify the etiology and source of infection in a diarrhea outbreak in Yunnan in May 2017 and to provide the evidence for formulating prevention and control measures. Methods: Epidemiological investigation was carried out on the epidemic situation of diarrhea in the village of Lvchun County in Yunnan Province, the field sampling, laboratory testing and data analysis were also performed. Results: Among the 44 patients, 11 of the 13 samples were positive for rotavirus nucleic acid in group A, and the positive rate was 84.62%. The survey showed that the water supply pipe was damaged and polluted by human and livestock manure and domestic sewage. The trend of the damaged water pipe was basically the same as the case distribution, and the rainfall was significantly related to the number of the disease. Conclusions This event was an outbreak of diarrhea caused by group A rotavirus. The direct pollution of drinking water caused by rainfall may be the risk factor of this outbreak. The health management of rural drinking water should be strengthened and the health knowledge and education of preventing intestinal infectious diseases should be promoted.