Abstract: The Yangtze River Economic Belt, a pivotal region for China’s high-quality economic development and ecological civilization construction, plays a strategic role in achieving sustainable growth. Addressing the limited understanding of policy diffusion mechanisms in this area, this study investigates the Yangtze River Protection Law through a policy diffusion lens, developing an integrated analytical framework that couples the Bass diffusion model with rough set theory to unravel spatiotemporal diffusion patterns and driving factors. Key findings include: (1)The policy diffusion exhibits a gradual sigmoidal curve, proceeding through three distinct phases: initial exploration (2019–2020), rapid expansion (2020–2022), and consolidation (2022–present), with internal imitation effects predominantly driving the adoption dynamics; (2) Spatially, a hierarchical diffusion pattern emerges, characterized by early adoption in core cities (e.g., Hubei, Chongqing) followed by neighboring regions; (3) Rough set analysis identifies industrial structure and technological capacity as the most critical drivers, surpassing administrative influence by over threefold. By innovatively merging policy diffusion theory with rough set methods, this work elucidates the multidimensional transmission mechanisms of basin-scale legal policies and proposes a dual-engine strategy prioritizing industrial and technological synergies. These insights establish an empirical foundation for a coordinated governance framework integrating legal constraints, industrial upgrading, and technological innovation, ultimately bridging the gap between institutional design and practical implementation in ecological governance.