Most offshore wind turbines at home and abroad use large diameter monopile foundation, and the control condition of large diameter monopile design is usually the response under horizontal load. The traditional API method is mainly used for slender piles, and the PISA method developed in recent years uses four kinds of springs to represent the reaction force of soil to piles, which is more suitable for monopiles with low aspect ratio in principle, but there is great controversy about the calculation of ultimate horizontal resistance in PISA method. The typical sand parameters in the South China Sea are selected for finite element analysis, and the key parameters such as soil relative density, pile diameter and aspect ratio are changed, and the response curves of pile foundation are compared. It is found that the three widely used expressions of ultimate horizontal resistance have obvious limitations. Based on the finite element analysis of a large number of variable parameters, it is suggested to express the change of ultimate horizontal resistance with depth in two stages.