To explore the relationship and mechanism of school loose-tight culture to middle school bullying, a total of 808 students were selected from three middle schools in Dehong Prefecture, Yunnan Province of China, to conduct a questionnaire survey. The study used the school loose-tight culture scale, the collective moral disengagement scale, the collective efficacy scale, and the bullying scale for middle school students. The results showed that: (1) school loose-tight culture significantly predicted the occurrence of school bullying; (2) school loose-tight culture was significantly negatively correlated with collective moral disengagement and school bullying but positively correlated with collective efficacy. Further, collective moral disengagement was significantly positively correlated with school bullying, but collective efficacy was significantly negatively correlated with school bullying; and (3) school loose-tight culture inhibited school bullying through the dual mediating effects of collective moral disengagement and collective efficacy at the same time.