The CO2 geological storage is an effective approach to mitigate global warming. The Baokang sedimentary system of Changling depression in the south part of Songliao Basin, choosen as the study area for CO2 storage capacity, is explored with sedimentary geology approach. A connected-well's profile map along the flow direction of Baokang System is investigated for the first time. The sedimentary facies, temporal and spatial distribution, sedimentary features, hydrogeology feature and teetonic characteristics of Baokang sedimentary system are studied based on observing cores and identifying rock thin sections under microscope. The results show that Qingshankou to Nenjiang Formation is favorable reservoir-caprock pair for underground CO2 storage and the total amount of potential CO2 storage capacity is estimated as 7. 43× 10^9 t, which is about two times the total volume of CO2 emission in China in 2002, or the total volume of CO2 emission in China in 2009.