Abstract: Objective To find out the cause,infection source and risk factors of a brucellosis epidemic in Tianzhu county of Gansu province in 2017 and provide evidence for the timely control of the epidemic. Methods Search related cases and conduct case investigation,consult clinical diagnosis and treatment data and laboratory test results. Results A total of 59 cases of human brucellosis were surveyed,86.44% (51/59) of cases occurred in domestic animal epidemic prevention. They participated in the immunization work of the live vaccine(S2)of sheep brucella in Tianzhu county from November to December, 2016. Participating in S2 vaccine strain immunization, the infection rate of brucellosis was 24.76%(51/206). The infection did not cause serious disease cases. The vaccination work did not comply with the biosafety regulations. Conclusion The outbreak of brucellosis was caused by contact with S2 vaccine strain. It is recommended that the animal husbandry and veterinary department should strengthen the protection knowledge of brucellosis,and carry out vaccination according to the biological safety regulation.