This study investigated the changes in social media use and cyber victimization before (November 2019) and during the COVID-19 pandemic (March 2020) among Chinese middle school students. It also examined the relation between cyber victimization and school adjustment overtime, and whether depressive symptoms mediated this relation and whether the social media use moderated this relation. We collected two waves of survey data from 651 seventh to ninth grade students (M[subscript age] = 13.93, SD[subscript age] = 1.17, 50.5% male) from two middle schools from Beijing, China over 4 months. Results indicated that middle school students spent more time on social media during the pandemic than before COVID-19 (d = 0.55). Cyber victimization was prevalent among Chinese middle school students at both time points (37.2% of students at T1 and 34.6% of students at T2 experienced some cyberbullying). The relationships between cyber victimization at T1 and T2 and school adjustment at T2 were fully mediated by depressive symptoms at T2. The indirect effects were -0.06 (mediation model, 95% CI [-0.12, -0.01], p = 0.02) and -0.07 (serial multiple mediation model, 95% CI [-0.11, -0.04], p < 0.001). Students' excessive social media use (more than 1 hr per day) also moderated this mediation. Specifically, for students who used social media excessively, cyber victimization at T1 directly predicted depressive symptoms at T2. However, this relation was not significant among students who used social media moderately (1 hr or less per day). These results highlight the importance of collaboration between educators and parents to monitor students' social media use, cyber victimization, and depressive symptoms in order to promote adjustment during COVID-19 virtual learning.