University students' engagement in online activities can impact both internet addiction and academic performance. This research aims to investigate these effects among university students. Using data from the "China College Students Longitudinal Survey" conducted by the Chinese Academy of Social Sciences between 2013 and 2018, this study examines the influence of university students' engagement in online activities. With a sample size of 15,102 university students, we explored the relationship between online activity, internet addiction, and academic performance. Employing stratified random sampling, the study investigates sample characteristics, including gender, study level, time spent online, and experiences accessing websites. Academic performance is assessed through six categories, employing descriptive and inferential statistics such as t-tests, ANOVA, stepwise linear regression, and path analysis. The findings indicate significantly high levels of university students' engagement in online activities, accompanied by a high prevalence of internet addiction. Key factors determining university students' engagement, internet addiction, and academic performance include gender, study level, time spent online, and experiences accessing websites beyond the Great Firewall. The study establishes a complex relationship, revealing negative effects of specific online engagements on internet addiction, while the latter influences university students' engagement. Overall, this research contributes valuable insights into the intricate interplay between online behaviors, addiction, and academic performance among Chinese university students, with implications for educational policies and interventions.