Recently, a groundbreaking advancement has emerged from the collaborative efforts of Researcher Du Chaohai from Peking University's School of Electronics and Professor Chen Hongsheng from Zhejiang University's College of Information Science and Electronic Engineering. Their innovative approach integrates phase gradient manipulation with Floquet periodicity principles, aiming to systematically uncover the previously overlooked harmonic dynamics inherent in the generalized Snell's law. By employing a deterministic Floquet-driven momentum compensation mechanism, they have achieved the selective activation of target harmonics. This breakthrough allows spatial harmonic components to be independently manipulated, paving the way for the development of a generalized Snell's law that is expanded to incorporate spatial harmonics. This new paradigm redirects the research trajectory of gradient metasurfaces, shifting from strategies focused on 'minimizing inter-unit coupling' to those that 'precisely harness strong inter-unit mutual coupling.' It opens up theoretical and practical avenues for the realization of ultra-dense beamforming techniques, reconfigurable multi-channel sensing systems, and the design of generalized metasurface devices. Furthermore, the research team has successfully demonstrated the practical application of this law through the fabrication and testing of three functional devices: a spatial harmonic-based anomalous unilateral reflector, a multi-beam splitter utilizing spatial harmonics, and a 'perfect' three-channel retroreflector boasting an impressive efficiency of up to 99%.