Abstract: The development and utilization of environment-friendly energy is imminent with global energy shortage and climate change becoming increasingly serious. The advancement and utilization of environment-friendly energy, including photocatalytic hydrogen production, electrocatalytic pollutant removal, and the development of electrochemical energy storage devices, have emerged as prominent research topics. A variety of modern analysis techniques provide strong support in these related studies, such as X-ray absorption spectroscopy (XAS) for investigating the active site geometry and electronic structure of photocatalysts, synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) for real-time monitoring of interatomic electron excitation and transfer in photocatalysts,in situelectron paramagnetic resonance spectroscopy (EPR) for the detection of free radicals in photocatalysis reactions, Raman spectroscopy (RS) for analyzing the interfacial structure of electrochemical reactions, andin situtransmission electron microscopy (TEM) for studying catalyst structure and reactivity in photocatalytic reactions. While these techniques provide abundant information for the mechanistic aspects of the associated chemical processes, it remains challenging to conduct further identification and structural analysis of the reaction intermediates at a molecular level. Due to the ability in rapid identification of the components in complex system, mass spectrometry (MS) has been widely used in the new energy fields such as photocatalytic reaction, electrocatalytic reaction, and the development of electrochemical energy storage devices. This review introduced the application of mass spectrometry techniques, including liquid chromatography-mass spectrometry (LC-MS), gas chromatography-mass spectrometry (GC-MS), matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), differential electrochemical mass spectrometry (DEMS), and electrospray ionization mass spectrometry (ESI-MS), in the detection of transient intermediates and products as well as the exploration of reaction mechanisms. Modern mass spectrometry offers critical support for the mechanism analysis of photocatalytic reactions aimed at the mineralization of organic pollutants, hydrogen generation, CO₂ reduction, N₂ fixation, and organic synthesis. It also provides distinct advantages for real-time product monitoring, isotopic labeling analysis, and the capture and identification of short-lived intermediates in electrocatalytic processes, such as hydrogen evolution, oxygen evolution, CO₂ reduction, N₂ reduction, and organic synthesis. By analyzing reaction products and intermediates in electrochemical energy storage devices through mass spectrometry, it is possible to elucidate the nature of energy storage and conduct rational design of high-performance materials. Modern mass spectrometry facilitates both offline detection of products and online identification of reaction intermediates, enhancing the understanding of reaction kinetics and elucidating mechanisms when combined with theoretical calculations. Thus, modern mass spectrometry has become a powerful tool for the study of reaction mechanisms. The challenges and prospects of mass spectrometry in the fields of photocatalytic degradation, photocatalytic synthesis, electrocatalytic reduction, electrocatalytic synthesis, lithium-ion batteries, and lithium-sulfur batteries were also discussed in this review. While current mass spectrometry techniques can detect transient intermediates, challenges persist in identifying the reaction intermediates with lower concentrations and reduced stability. Therefore, optimizing the sampling systems and mass analyzers is crucial to improve the sampling speed, transfer efficiency, and analysis throughput. Understanding complex reaction mechanisms requires correlating reaction intermediates with various conditions, such as reaction time, radiation wavelength, and potential. This necessitates a spatial integration between the reactor and the mass spectrometer to preserve the true form of intermediates and ensure accurate signal transmission that reflects the pertinent reaction parameters. Integrating mass spectrometry to analyze the morphology and structure of reaction intermediates and products with theoretical calculations may enable the construction of a comprehensive, multi-dimensional reaction theory system. Ultimately, employing this theoretical framework to advance experimental research could represent a novel idea in the research filed.