Coach-athlete relationship (CAR), thriving and athlete engagement are important psychological variables that affect sports performance. On the basis of self-determination theory, this study constructs a mediation model to examine the influence of CAR on athlete engagement and the mediating effect of thriving between them. This cross-sectional study involves a questionnaire survey with 287 Chinese active athletes (M = 19.63, SD = 2.53) aged 14-26 years (64.5% male, 35.5% female) from eight sports. CAR, thriving and athlete engagement were assessed using the CAR Questionnaire, the Thriving Scale, and the Athlete Engagement Questionnaire, respectively. CAR and its dimensions can significantly and positively predict athlete engagement, complementarity, commitment, and closeness, accounting for 35.1%, 34.6%, and 30.4% of the cumulative variance in dominance analysis, respectively. The direct and indirect paths show that CAR affects athlete engagement through the mediating effect of thriving. The mediating effect model has a good fit and indirect effects account for 56.9% of the total effects. The effect of CAR on athlete engagement reflects a practical application of interpersonal dynamics in competitive sports to a certain extent. The following suggestions can be used to improve athlete engagement. First, setting common goals, emphasizing mutual cooperation, and building trust and support, promote coaches and athletes to have a higher sense of commitment and complementarity to each other, thereby helping improve athlete engagement. Second, meeting the vitality and progress needs of athletes effectively mobilizes CAR resources to promote athlete engagement, which can be manipulated by cultivating closeness, commitment, and complementarity. Third, to ensure the athletes' sports state and mental health, the sports team should focus on the cultivation of athletes' capacities to thrive and internally form a dynamic and positive sports atmosphere in their team. In the future, we can track and compare the influence of the improvement of CAR on thriving and athlete engagement can be tracked and compared from the dual perspectives of coaches and athletes.