Significance: With the rapid development of routine launches, deep space exploration, lunar and Mars missions, and commercial spaceflights, increasingly stringent demands have been placed on the safety, reliability, and efficiency of propulsion systems. Cryogenic propellants, such as liquid hydrogen, liquid oxygen, and liquid methane, have become core working fluids for next-generation launch vehicles and deep space missions owing to their high specific impulse and clean combustion products. However, under conditions of a high flow rate and rapid fuel, severe water hammer and complex transient flow phenomena are likely to occur in propellant feed lines; these phenomer include large-amplitude pressure oscillations, rapid multiphase transitions, and strong coupling between thermal and structural fields. If uncontrolled, such nonequilibrium transient processes may induce pipeline vibrations, valve malfunctions, tank structural damage, or even catastrophic failures. Therefore, elucidating the mechanisms of water hammer during the rapid filling of cryogenic propellants is not only of great scientific importance but also an urgent requirement for ensuring spacecraft mission safety and improving the design of modern launch sites. Progress: In recent years, extensive research has been conducted worldwide on the transient flow characteristics of cryogenic propellant filling, leading to a series of significant advances. Theoretical modeling has evolved from traditional one-dimensional water hammer equations to multiphase flow models that incorporate thermodynamic nonequilibrium effects, phase-change dynamics, and interfacial heat transfer, thereby providing a more accurate representation of cryogenic operating conditions. Numerical simulation methods, including high-resolution finite-volume methods, multiscale direct numerical simulations, and coupled fluid-thermal-structural approaches, have been developed, enabling more accurate simulations of pressure oscillations, gas-liquid interface evolution, and heat transfer phenomena. In terms of experimental validation, given the stringent requirements during liquid hydrogen and liquid oxygen testing, researchers have employed substitution experiments with liquid nitrogen, liquid helium, and cryogenic simulation test facilities to obtain key transient data; some of these experimental results have already been successfully applied to assess the safety of rocket ground fuel systems. From an engineering perspective, organizations such as the National Aeronautics and Space Administration (NASA) and SpaceX have introduced energy dissipators, buffer devices, and active control strategies into next-generation launch site construction to effectively suppress local cavitation, flashing, and water hammer risks. Collectively, these efforts indicate that the research focus is gradually shifting from single-flow modeling to an integrated development paradigm, encompassing mechanistic understanding, model development, experimental validation, and engineering applications. Conclusions and Prospects: Despite these advances, critical challenges remain in the study of cryogenic propellant filling. First, the nonequilibrium nature of phase-change processes under cryogenic conditions still lacks a complete and unified quantitative description. Second, the mechanisms of fluid-thermal-structural multiphysics coupling are highly complex, and the current models show limitations in terms of boundary adaptability and predictive accuracy. Third, experimental validation remains limited, highlighting the urgent need for safe and controllable substitution methods and advanced testing platforms. Future research should establish a closed-loop framework of "mechanistic understanding-model development-experimental validation", promote the integration of high-fidelity multiphase flow models with multiscale simulation techniques, and leverage artificial intelligence and big data approaches to achieve intelligent prediction and real-time control of transient fueling processes. The coordinated advancement of theory, experimentation, and engineering practice in this field is essential to provide solid theoretical support and technological assurance for the safe implementation of next-generation routine launch operations and high-frequency mission scenarios.