Abstract Within the framework of positive youth development (PYD) and life course theory, this study was designed to examine patterns of PYD and promotive factors over the first semester of Chinese high school with a sample of 480 students (boys, 43.96%). The growth mixture model identified four trajectories of PYD, labeled high start–fast decreasing, high start–low decreasing, low start–low increasing, and mid–persistent. Results also showed that membership in a PYD trajectory is significantly related to three promotive factors: parental involvement, teacher involvement, and intentional self‐regulation. These findings have important theoretical and practical insights for a deeper understanding of the underlying mechanisms of positive development in adolescents following the critical period surrounding the transition to high school.