In an effort to detect and quantify potential CO₂ leakage from geologic storage sites, this work investigates the migration process of CO₂ leaked in a shallow subsurface associated with the CO₂ sequestration project at Shaanxi, China. A shallow subsurface model with aquifer and soil layers was established, and the sensitivities of leakage rate and geological conditions were analysed. The results show that the distribution morphology of the CO₂ plume is generally mushroom-shaped when the leaked CO₂ enters the formation as a point source. Maximum leakage occurs at the soil surface due to the comprehensive effects of pressure, concentration, and density differences between CO₂ and soil gas. The mass fraction of CO₂ in gas is approximately 0.38–0.48 near the surface leak point, and leaked CO₂ moves laterally along the bottom of the soil vadose zone. The leakage rate of CO₂ significantly affects the pressure build-up as observed for an increase in the CO₂ leakage rate from 0.0002 to 0.02 kg/s. In addition to the loess beam area, the migration process of CO₂ leakage in the valley terrace area was also studied. The results show that the CO₂ concentration in the leakage centre area of the valley terrace is not much different from that in the loess beam. Moreover, the surface leakage range in the valley terrace is much smaller than that in the loess beam, which is because of the differences in the formation composition and thickness. Heterogeneity has limited effect on leakage.