Abstract: Objective Retrospective analysis was conducted for a tuberculosis (TB) outbreak in a middle school to explore the best principles and methods of handling the TB outbreak.Methods A total of 229 students in grade 3 in the junior high school with TB outbreak were screened by chest X ray and PPD. The distribution of time, space and population was described, and the relation among the exposure level, TST result and the secondary cases was evaluated.Results According to the degree of contact with the index TB case, the 229 students were divided into four groups: Level 1 (L1) students studied with the index case in the same classroom. Level 2 (L2): students studied in the adjacent class but studied frequently in the same classroom. Level 3 (L3) students studied on the same floor but in different classroom with the index case. Level 4 (L4) students studied on the different floor and in different classrooms. Based on the above criteria, the strongly positive PPD rates were 74.0% (37/50), 58.5% (24/41), 31.8% (21/66) and 12.7% (9/71) respectively in the 4 group. The attack rates of TB were 16% (8/50) and 2.4% (1/41) in L1 and L2 respectively (χ²=4.65, P<0.05). According to the TST results of the first screening in close contacts (L1), the attack rates of TB of the group of TST≥15 mm and TST＜15 mm were 13.3% (2/15) and 17.1% (6/35) respectively (χ²=0.11, P>0.05). Based on the TST results of the second screening in close contacts (L1, 3 months later), the attack rates of TB with TST≥15 mm and TST＜15 mm were 16.2% (6/37) and 15.4% (2/13) respectively (χ²=0.00, P>0.05). In the TB outbreak, even if the preventive therapy was applied to all of the 15 students with TST≥15 mm after the first TST, only 22.2% (2/9) of the se-condary case could be selected to received the early intervention.Conclusion The level of exposure is a high risk factor for TB incidence during an TB outbreak. Whether the TST is strongly positive or not is unrelated to secondary cases. Establishing a mechanism for risk evaluation is an effective means of preventing TB outbreak in school.