To reveal the influence of delayed-peak rainfall infiltration on ultimate bearing capacity of an unsaturated foundation rapidly, the following research work was carried out. The Gardner’s permeability coefficient function and the soil-water characteristic curve model were introduced to simplify the Richards unsaturated seepage control equation, and the Laplace transform was adopted to derive the transient analytical solution of matrix suction for foundation under delayed-peak rainfall infiltration with rain intensity, which was verified with the finite element numerical simulation results. Based on the theory of shear strength for unsaturated soil and the transient analytical solution of matrix suction, the expression of transient ultimate bearing capacity for foundation was obtained under delayed-peak rainfall infiltration. The mapping relationship between the ultimate bearing capacity and the initial rain intensity, the rain intensity variation coefficient, and the rainfall duration was established. With the analytic solution of the ultimate bearing capacity, the evolution characteristics of the bearing capacity for foundation were studied during the delayed-peak rainfall, and the effects of the initial rain intensity and the rain intensity coefficient on the ultimate bearing capacity for foundation were analyzed. The results show that during delayed-peak rainfall infiltration, the ultimate bearing capacity decreases gradually with a nonlinear velocity from low to high, which is different from the near linear reduction when encountering uniform rainfall. When the rain intensity coefficient is constant, ultimate bearing capacity for foundation decreases faster with the greater initial rain intensity. When the initial rain intensity is constant, the ultimate bearing capacity for foundation decreases faster with greater rain intensity variation coefficient.