Abstract: Synchrotron radiation technology, leveraging its exceptional advantages in high brightness, superior spatiotemporal resolution, and strong penetration capability, has revolutionized the real-time characterization of dynamic internal behaviors in molten pools during welding and metal additive manufacturing processes. This review systematically summarizes the advancements in synchrotron radiation applications across critical domains, including gas-liquid interface dynamics, solid-liquid interface evolution, metal flow behaviors, and bubble dynamics. Research demonstrates that synchrotron radiation overcomes the spatiotemporal resolution and penetration depth limitations of conventional characterization methods, thereby elucidating multiphysics coupling mechanisms within molten pools and defect formation principles. It offers high-precision experimental benchmarks for process parameter optimization and numerical model validation. Furthermore, integrating artificial intelligence with synchrotron radiation has significantly enhanced data analysis efficiency. However, current techniques still face challenges such as insufficient spatiotemporal resolution, limited multimodality synergy, and a lack of full lifecycle observation capabilities. Looking forward, the popularization of fourth-generation synchrotron radiation light sources, the advancement of multi-modal correlative techniques, and the deep integration of artificial intelligence will drive comprehensive-cycle, multi-scale, and high-fidelity advancement, thereby enhancing process optimization and industrial applications in fusion welding and metal additive manufacturing.