In this paper, based on Meyerhof’s theory of homogeneous foundation, the limit equilibrium analysis method and unified logarithmic spiral sliding surface assumption are used to derive the theoretical formula for the ultimate bearing capacity of a layered foundation when the foundation is completely rough. It should be noted that this formula is only applicable to strip foundations of upper soft clay and lower sandy soil. In addition, a comparative analysis is conducted between theoretical formulas and semiempirical formulas for layered foundations. On the basis of verifying the reliability of the theoretical formula results, numerical simulation is carried out to further explore and analyze the influence of the width to depth ratio of the foundation, the strength parameters of the double-layer soil, and the thickness of the upper soft soil on the bearing capacity of the foundation. Research has shown that the formula for the bearing capacity of a layered foundation derived in this paper has a certain degree of error compared to Meyerhof’s semiempirical formula, but it is in good agreement with numerical simulation results and Hansen’s weighted average method results. The ratio of the width to depth of the foundation, the ratio of the cohesive force of the double-layer soil, and the tangent ratio of the internal friction angle have a significant positive correlation with the ultimate bearing capacity of the foundation. The increase in thickness of the overlying cohesive soil has a negative impact on the ultimate bearing capacity of the foundation, and the thicker the soil, the smaller the foundation’s bearing capacity.