Abstract: Objective Due to its high density, supercritical/dense CO₂ may cause transient water hammer during pipeline transportation when valves are closed for planned or emergency shutdowns. Currently, there is a lack of experimental research on water hammer in CO₂ pipelines. Methods: A transient water hammer test system for supercritical/dense-phase CO₂ pipelines was designed and constructed, featuring a 238-meter length, 56-mm diameter, and a design pressure of 20 MPa. Fifteen transient water hammer experiments were conducted to analyze the response patterns of supercritical/dense-phase CO₂ water hammer and validate theoretical models and OLGA simulation models. 【Results】At identical initial temperatures, increased initial pressure significantly amplified water hammer pressure amplitude while shortening the water hammer cycle. Conversely, at the same initial pressure, elevated temperatures reduced CO₂ density and sound velocity, thereby diminishing water hammer pressure amplitude and prolonging the cycle. Increased CO₂ flow velocity within the pipe and shorter valve closure times result in higher system energy at high velocities. The more intense kinetic energy release caused by rapid valve closure significantly exacerbates water hammer intensity. However, CO₂ flow velocity and valve closure time do not affect the water hammer cycle, which is primarily controlled by initial temperature and pressure. When designing and operating pipeline systems, priority should be given to mitigating water hammer overpressure risks. Appropriately reducing operating pressure and increasing operating temperature can lessen transient pressure shocks caused by water hammer. Additionally, rapid valve opening or closing operations should be avoided during high flow rate conditions. Conclusion This study systematically investigates the response characteristics and influencing factors of valve-closing water hammer in supercritical/dense-phase CO₂ pipelines through experimental validation, providing crucial evidence for establishing supercritical CO₂ water hammer models and guiding CO₂ pipeline design and protection.