Storage of carbon dioxide is one of the effective methods for utilizing greenhouse gas and reducing greenhouse gas discharge. The storage of CO₂ into saline aquifers is the best way to mitigate the green house effect. However, there is a risk of leaking during the injecting of CO₂ into saline aquifers. So it is necessary to calculate the displace radius of CO₂ in saline aquifer. Based on the Buckley-Leverett equation, a new formula is derived to calculate the displace radius of CO₂ in saline aquifer. The research results can provide a theoretic support and a field guide for calculating CO₂ storage volume and risk control in saline aquifer.