Against the background of China’s carbon peaking and carbon neutrality goals and the rapid development of carbon capture, utilization and storage (CCUS) technologies, the subsurface migration and surface hazardous range caused by leakage from buried CO? pipelines are closely related to leak detection, risk assessment, and emergency response. In this study, a three-dimensional CO? leakage and dispersion model was established using COMSOL Multiphysics, and HYSYS was employed to analyze the phase behavior of CO? mixtures containing impurities. Sandy soil, a burial depth of 1.5 m, an orifice diameter of 8 mm, a circular leakage orifice, and impurity-free CO? were selected as the baseline conditions. A one-factor-at-a-time method was used to investigate the effects of leakage orifice diameter, orifice shape, burial depth, soil type, and impurity gas. The results showed that a larger orifice diameter and a shallower burial depth led to a faster increase in CO? concentration at the monitoring points and an earlier formation of a larger surface hazardous area. Under the same equivalent diameter, the rectangular-orifice case produced the largest hazardous diameter. CO? dispersed fastest in sandy soil, followed by loam and clay. Both CH? and N? promoted CO? dispersion, with CH? exerting a stronger influence. The relative influence magnitude of each factor was evaluated based on the variation range of the hazardous diameter at 600 s relative to the baseline condition. The resulting order was pipeline burial depth > leakage orifice diameter > soil type > impurity gas > leakage orifice shape. These results represent the conditional main effects of individual factors within the specified parameter ranges and can provide a reference for leak detection and risk assessment of buried CO? pipelines.