Aims: This study aimed to investigate the prevalence of anxiety, depression and PTSD symptoms, and associated risk factors among a large-scale sample of adolescents from China after the pandemic and lockdown. Method: A total of 57,948 high school students took part in an online survey from July 13 to 29, 2020. The mental health outcomes included anxiety, depression and PTSD symptoms. Risk factors included negative family relationships, COVID-19 related exposure, and a lack of social support. Results: The prevalence of anxiety, depression and PTSD symptoms was 7.1%, 12.8%, and 16.9%, respectively. COVID-19 related exposure significantly linked to the mental health outcomes (all p < .001). The most important predictors for the mental health outcomes were family relationship and social support (all p < .001). Conclusion: The pandemic may have long-term adverse mental health consequences among adolescents. Adverse family relationships and lack of social support could be the major risk factors for the post-pandemic mental health outcomes of adolescents.