The uplift pressure is one of the main factors that influence the stability of underground structures.For the calculation of uplift pressure on clay foundation,traditional method considers that the uplift pressure works on the whole area of the base.It does not give considerations of pressure decreasing caused by the close contact between clay particles and the base.Through detailed analysis,the present article considers that:for clay foundation,the concept of effective sectional void ratio can be introduced.Hence,the value of uplift pressure is smaller than those calculated by traditional methods,which can save the investment of the projects under the circumstance of safety.