Abundant evidence highlights the benefits of self-determined motivation (e.g., parental autonomy support as internalized extrinsic motivation, gratitude as intrinsic motivation) on academic engagement during adolescence, yet the potential mutual relations remain relatively unexplored. This study investigated the bidirectional relations and potential mechanisms among parental autonomy support, gratitude, and academic engagement using a traditional cross-lag-panel model (CLPM) and a within-person CLPM with random intercept (RI-CLPM) in a sample of Chinese youth (N = 1214; Mage = 15.46, SDage = 0.71; 39.30% girls) across three time points with 6-month intervals. The results indicated that the bidirectional relation between parental autonomy support and academic engagement was present in CLPM but not in RI-CLPM. However, the bidirectional relations between gratitude and academic engagement, and between gratitude and parental autonomy support, existed at both levels. Furthermore, in CLPM, parental autonomy support influenced academic engagement through gratitude, and gratitude, in turn, affected academic engagement through parental autonomy support. Academic engagement impacted gratitude via parental autonomy support, and simultaneously, academic engagement influenced parental autonomy support through gratitude. Academic engagement served as a mediator between parental autonomy support and gratitude, as well as between gratitude and parental autonomy support. Five self-enhancing loops were identified in CLPM. These findings reveal a virtuous cycle of mutual influence between parental autonomy support, gratitude, and adolescent academic engagement, highlighting the important role of academic engagement in strengthening autonomous motivation.