Abstract: The international popular API code recommends the p-y curve method to analyze the nonlinear behavior of offshore wind turbine (OWT) steel pipe piles. The p-y curve controls the pile-soil nonlinear response only through one parameter regarding the development of stress-strain relation of soils. This over-simplification results in the inaccurate evaluation of the lateral initial stiffness of pile-soil and the underestimation of the bearing capacity. Therefore, the stress-strain curve with soil small-strain behavior is first introduced to achieve a numerical p-y backbone curve by using the total-displacement-loading extended mobilized strength design method (T-EMSD). The expression for the two-dimensional p-y backbone curve is then fitted from the numerical results. The three-dimensional effect of the proposed p-y curve is further considered by incorporating the three-dimensional ultimate capacity factor, the initial subgrade modulus and the compatibility factor. The rationality of the proposed p-y curve is verified against the results from the three-dimensional finite-element analysis and field tests. Compared with API code, the proposed p-y curve can provide a more reasonable prediction for both the bearing capacity and the initial stiffness of pile-soil by considering the soil small-strain behavior, which is a significant advantage for the OWT pile foundation with strict deformation control.