Background: The COVID-19 pandemic has brought a new threat to child health and safety. Some studies suggest that social isolation and economic stress have exacerbated child abuse and neglect, whereas other studies argue that orders to stay at home are likely to promote parent–child relationships during this stressful time. Due to a lack of prospective studies including before–during–after lockdown assessments, the impacts of lockdown measures on child maltreatment are unclear. Methods: This study retrospectively investigated child maltreatment of 2821 Chinese children and adolescents from 12 to 18 (female, 59%) before, during and after lockdown, and identified risk factors. Potential predictors including socio-economic and individual mental health status were collected. Results: During Chinese lockdown, children and adolescents reported that the proportions of decrease (range 18–47.5%) in emotional abuse and neglect, physical abuse and neglect, sexual abuse, and witnessing domestic violence were greater than that of increase (range 5.1–9.1%). Compared with before lockdown (1.6%), the prevalence of sexual abuse significantly increased 8 months (2.9%) after the lifting of lockdown (p = 0.002). Being male, suffering from depression, state anhedonia, and experiencing psychotic symptoms at baseline were associated with increased sexual abuse after lockdown. Conclusions: The impact of lockdown on child maltreatment was beneficial in the short-term but detrimental in the long-term in China.