The assessment of carbon dioxide geological storage in deep saline aquifer in the central depression of Songliao Basin is taken as an example in this paper. In the paper, the authors created 1 000 random numbers of porosity to calculate the carbon dioxide geological storage capacity and analyse the change caused by the uncertainty of the porosity. The result shows that the theoretical storage capacity of the deep saline aquifer in the central depression of Songliao Basin is 5.34×1011 t, which is between 4.14×1011 t and 5.72×1011 t when the uncertainty of the porosity is considered and the cumulative probability is 58.3 % of the storage capacity which is larger than theoretical storage capacity. It shows that the uncertainty of the porosity has great effect on the result of the assessments, which should be taken into account in actual projects.