Based on the field test of horizontal loading characteristics of a single pile in an offshore wind farm, the load-displacement response, pile deflection and bending moment of a 2.4 m large-diameter steel-pipe pile in deep soft clay groundsill were studied. The applicability of m method, p-y curve and hyperbolic curve recommended by the API specification for soft clay foundation is discussed. Numerical models were established to analyze the contribution factors of horizontal bearing capacity of single pile foundations with different diameters. The results showed that the initial stiffness of pile and soil resistance in the shallow soil p-y curve from the API specification method and the hyperbolic is larger, and the hyperbolic under a certain depth can better predict the changing relationship of soil reaction force with displacement. The calculated horizontal displacement of m method and hyperbola is smaller than measured values, tending to be unsafe results, while the calculated results of the API specification method are larger compared with the measured values, more conservative. With the increase of pile diameter, the contribution of lateral friction and foundation resistance to the horizontal bearing capacity of single pile foundation will also increase, and the p-y curve method will seriously underestimate the horizontal bearing capacity of single pile.