On the condition of sand overlying soft clay, it is difficult to determine the ultimate bearing capacity of foundation. The current computation methods have several limitations, which may cause large deviation. On the basis of the spread projection model, the computation formula for bearing capacity of strip footing on sand overlying soft clay is deduced according to stress analysis of sand element beneath the footing. The precision of the proposed formula is validated by comparison with centrifuge test data in the literature. The numerical model of two-layer foundation with sand over soft clay is generated, and the validation is conducted through comparing with the results of existing experiments and numerical simulation. Then the analysis of main parameters in the formula is carried out using the numerical model. The results indicate that the normalized strength of clay and dilation angle of sand have significant influence on the failure mode of soil foundation. In the case that c_u/γB ≤ 2 and the value of spread angle is determined by the proposed formula, the precision of the theoretical formula is satisfactory. The soil moduli ratio makes little effect on the bearing capacity.