In regions with strong earthquake disturbances, sufficient landslide sediments would activate debris flows for decades. As of 2024, although the Wenchuan earthquake has occurred 16 years ago, the nonuniform prolonged variation of post-quake debris flows is not fully understood owing to a lack of sufficient long-term monitoring data. In this study, a new debris flow activity index (DFAI) considering the effects of the landslide sediment supply, rainfall and topography, was proposed. The results of the DFAI reveal that post-quake debris flow activity continues to decline in a power pattern form and approximately decline toward the preseismic level for 50 years. The debris flow activity level was categorized into low, slight, medium, high and extreme levels on the basis of the DFAI and field investigations. The results of the debris flow activity assessment revealed that many debris flows experienced relatively high activity between 2008 and 2013, and gradually evolved from extreme to high levels at a rate of 0.06 yr- 1. In contrast, debris flow activity rapidly declined after 2013, and changed from extreme and high levels to low, slight, and medium levels at rates of -0.06 yr- 1 and -0.02 yr- 1, respectively. Presently, approximately 59.32 % of the catchment has low and slight debris flow activity, but only 10 % of the catchment has high and extreme debris flow activity. The regions with higher debris flow activity have changed to the northern regions of the study area, and the larger catchment has relatively high debris flow activity. In addition, we validated the performance and robustness of the DFAI at sites with different geological environments and climates. In all, the DFAI can reduce the limitation of insufficient monitoring data and has good performance in reflecting nonuniform prolonged changes in debris flow activity.