Abstract: In recent years, research on light-matter interactions based on cavity quantum electrodynamics (QED) has advanced beyond traditional frameworks with the emergence of the deep integration mechanism known as ultra-strong coupling. In this regime, light and matter are no longer merely independent entities exchanging energy; instead, they are “bound”together through strong interactions, forming an inseparable hybrid quasiparticle—the polariton—which possesses the dual attributes of both light and matter, the system can exhibit novel physical properties, such as ground state entanglement and virtual excitations. Various experimental platforms, including semiconductor quantum wells, superconducting circuits, and cavity magnonic systems, have successively achieved and observed the ultra-strong coupling effect, providing critical validation for theoretical predictions. The theory of cavity QED in the ultra-strong coupling regime has not only propelled advances in cutting-edge fields such as quantum information processing, precision measurement, and topological photonics, but its focus is also gradually shifting from the observation and characterization of phenomena toward the effective control and functional application of coupled states. This progression foreshadows its even more profound impact on fundamental science and interdisciplinary applications.