Abstract: Compared with traditional pile foundations, offshore monopile foundations are featured by their large pile diameters (D>3) and low aspect ratios (L_m/D<10), and usually exhibit semi-rigid deformation characteristics under lateral loads. The difference in pile-soil relative stiffness leads to the fact that the traditional design methods, such as the p-y curve method of API specification, are not suitable for the current offshore monopile. To investigate the effect of pile-soil relative stiffness on the p-y curve, a three-dimensional numerical analysis of large-diameter monopiles under lateral loads is carried out using the finite element software ABAQUS. Firstly, the distribution characteristics of the soil reactions around the pile along the circular and vertical directions are clarified. Secondly, the effects of the pile diameter D, of the pile aspect ratio L_m/D, and of the soil elastic modulus E_s on the initial stiffness of the p-y curve are analyzed, and the initial stiffness model of the p-y curve considering the pile-soil relative stiffness is obtained by the equivalent pile head stiffness method. Then, a function is fitted to the distribution of lateral soil reactions around the pile to obtain the calculation model of the ultimate soil resistance. Finally, one new p-y curve model for the large-diameter monopile is established by adopting hyperbolic function as the backbone curve. The results show that the initial stiffness of the p-y curve is independent of the pile diameter and increases with the increment of the pile-soil relative stiffness. The relative pile-soil stiffness is affected by different growth patterns of soil E_s along the burial depth, which need to be taken into account in the analysis. The p-y model established has good applicability and can provide an important reference for the design of large-diameter monopiles in sand.