Abstract <p>This study designed a visualization experimental system for investigating the flow characteristics and phase transition processes during small-hole leakage of supercritical CO₂ pipelines. This experimental system introduces innovations in the test pipeline, observation window, and release valve assembly, which expand the observation range and improve the universality of the experimental conclusions. In this study, experiments on supercritical CO₂ release were conducted at various initial temperatures using this system. The results indicated that the evolution process of supercritical CO₂ leakage can be categorized into three stages: supercritical, vapor-liquid phase, and vapor phase. During the transition from supercritical to vapor-liquid phase, the depressurization rate significantly decreased, leading to the formation of an inflection region on the depressurization curve. The initial temperature notably influenced both the pressure level and duration of the inflection region. When the initial temperature exceeded 55°C, the inflection region disappeared, and the release process of supercritical CO₂ became similar to that of vapor CO₂. These experiments also demonstrated the reliability of the experimental system.</p>