The monopile is one of the most common solutions for the foundation of offshore wind power plants. The p-y curve recommended by API is commonly used to obtain the horizontal displacement of a monopile. For piles of a more than 2 m diameter, the p-y curve model overestimates the initial stiffness of subgrade reaction and yields inaccurate horizontal displacement. An evaluation of the current p-y method was made to obtain the key factors responsible for overestimating the initial stiffness of subgrade reaction. These key factors were amended. A modified p-y curve model was proposed in which the nonlinear impact of the dimeter and depth on the initial stiffness K was considered based on the calculation results of FLAC3D. The developed p-y curve model was compared with test data and the results of the current p-y curve and the previous modified model. The results show that the good agreement between the calculated results of the developed modified p-y model and test values verified the rationality of the modified p-y model.