Background and Objectives: As problematic internet use (PIU) becomes increasingly prevalent among university students, effective preventive measures remain scarce. This study aimed to investigate how the allocation of daily activity time influences PIU and PIU risk (PIU/PIUR) and to identify specific activities that serve as risk and protective factors along with their effect strength. Methods: Data from 2,433 university students in 33 Chinese provinces were analyzed using compositional analysis, isotemporal substitution, and instrumental variable methods to determine causal relationships between activity allocation and PIU/PIUR and to calculate the specific effects of substituting one activity for another. Results: After compositional adjustment, moderate-to-vigorous physical activity (MVPA) and classroom learning statistically significantly reduced PIU/PIUR (ps < 0.001, except PIUR for classroom learning: p 5 0.002), whereas short videos and gaming increased PIU/PIUR (ps < 0.001). Sleep (PIU: p 5 0.023, PIUR: p 5 0.009) and autonomous learning (PIU: p 5 0.013, PIUR: p 5 0.003) were negatively correlated with PIU/PIUR but had no significant causal effect. Light physical activity was not statistically significantly correlated with PIU/PIUR (PIU: p 5 0.141, PIUR: p 5 0.585). Substituting 30 min of short video time with MVPA reduced PIUR by 22.9%. Conversely, replacing MVPA with short video watching increased PIUR by 68.3%. Discussion and Conclusions: Findings demonstrate the significant impact of 24-hour activity allocation on PIU/PIUR and suggest that time allocation strategies, particularly increasing MVPA while reducing short videos time, effectively reduce PIUR. These insights identify potential prevention for managing PIU via reallocation of daily activities.  Kulcsszavak: daily activity allocation, problematic internet use, compositional data analysis, university students, internet addiction