Abstract: CO2 capture, utilization and storage(CCUS) is one of the important technical ways for greenhouse gas reduction. The sealing capacity of caprocks is an important index for screening CO2 storage site, which has one ticket veto. Based on the knowledge of sealing mechanism, the physical sealing capacity of caprocks is defined in the context of CCUS.According to the assessment thought of oil/gas reservoirs, the maximum CO2 plume height is chosen as the quantitative index for sealing capacity assessment. In consideration of mechanical equilibrium between CO2 buoyancy and capillary pressure of caprocks, the maximum CO2 plume height is calculated for Jilin oil field as a case study. This paper provides a way for quantitative assessment of effective CO2 capacity before the CO2 breakthrough.