Electrocatalytic water splitting represents a sustainable and efficient approach for producing high-purity hydrogen, playing an increasingly pivotal role in addressing global energy sustainability challenges. However, dynamic and complex electrocatalytic processes pose significant obstacles to unraveling electrocatalytic mechanisms and advancing catalyst design. This review first discusses fundamental principles for conducting reliable in situ/operando synchrotron radiation (SR) spectroscopic measurements in electrocatalytic systems, proposing guidelines for standardizing practices across the community. Then, cutting-edge in situ/operando SR-based spectroscopic techniques applied in electrocatalytic water splitting are systematically examined, highlighting their distinctive advantages while critically evaluating inherent methodological limitations. Moving beyond conventional single-technique approaches, we focus on complementary probes based on in situ/operando multi-SR spectroscopic technologies to achieve panoramic visualization of the dynamic evolution for the water splitting process, spanning from the atomic and molecular scales to the electronic level. Finally, key bottlenecks and frontier research opportunities are outlined, aiming to inspire a paradigm shift from fragmented analysis toward integrated, system-level mechanistic understanding in electrocatalytic water splitting.The standardized principle of in situ/operando synchrotron radiation characterization has been proposed, and multiple technical probes have been integrated to achieve panoramic and multiscale visualization of water splitting, guiding the future of rational catalyst design.