Abstract: The paper was designed to conduct field epidemiological survey and analysis on an hepatitis A outbreak after spring outing among the students of a middle school in Dongbo township of Yao ethnic group in Funing county. Descriptive epidemiological analysis and case control study were conducted to identify the risk factors by taken 26 hospitalized students of the middle school who were diagnosed as hepatitis A during 22 April-11 May, 2008 as the case group and 52 non-sick classmates of same gender and without receiving hepatitis A vaccination as the control group. The attack rate was 29.51% among the students, and the case-control study indicated that the outbreak of hepatitis A was associated with drinking untreated water from mountain stream during the spring outing (2=4.551, P=0.033），OR=4.865（ORU=23.204，ORL=1.020）. The difference on the incidence among the students who went to sightseeing to same place and drinking the water at different time (late March and late April) showed statistical significance (2=16.201,P=0.000). The outbreak of hepatitis A was associated with drinking the untreated stream water which was once polluted.