This paper analyses laboratory model tests of double -layer subgrade and investigates the influences of changes in the thickness of soft clay overlying hard soil layer and water-cement ratio on the ultimate bearing capacity of soft soil foundation overlying hard soil layer. It is shown that the increased water-cement ratio has a remarkable impact on the increases in the ultimate bearing capacity, while increases in the thickness do not have obvious effects on the increase of the ultimate bearing capacity of double-layer subgrade. Furthermore, this paper analyses the ultimate bearing capacity of double-layer subgrade, puts forward the amendment formula of the ultimate bearing capacity, and carries out a comparative analysis between the calculated results and the experiment data.