ABSTRACT During the COVID-19 pandemic, internet use and gaming of adolescents had been elevated. On the one hand, internet use and gaming in the period was a good approach to killing quarantined time. However, the increased use of the internet and game of adolescents may also increase the risk of internet addiction. This study aimed to describe the internet behavior changes of adolescents and to understand the impact of clinical features on internet addiction after the adolescents back to school in COVID-19 period. We conducted a cross-sectional cohort study using data collected through online investigation in China. Six hundred and twenty-five adolescents completed the online survey. Results indicated that internet addiction, having gaming use behaviors, Strengths and Difficulties Questionnaire (SDQ) Conduct subscale scores and the SDQ Prosocial subscale scores before the COVID-19 outbreak were significant in predicting the internet addiction after the adolescents back to school. The proportion of internet addiction and gaming behaviors among adolescents in China was changed across the COVID-19 pandemic. Clinical features before the COVID-19 pandemic could predict Internet addiction risk after the adolescents back to school.