This study examined the longitudinal relationship between coping styles and sleep quality, as well as the underlying mechanism from the perspective of sleep-competing activities among high school students. We used three-wave longitudinal data collected over one academic semester (representing a complete learning cycle) from a sample of 515 Chinese adolescents aged 14 to 18. Cross-lagged panel model (CLPM) revealed that baseline negative coping style (T1) was associated with subsequent changes in sleep quality (T3), and positive coping style (T1) was associated with better sleep quality (T1) subsequently. Longitudinal mediation analysis revealed that sleep competing activities at T2 mediated the relationship between negative coping style at T1 and sleep quality at T3, as well as the relationship between positive coping style at T1 and sleep quality at T3. These findings may have implications for developing targeted interventions to improve adolescent sleep quality, particularly through coping skills training and sleep hygiene education.