Abstract: In order to meet the requirements of construction and bearing capacity, the measure of the variable diameter of the pile body is often used for the monopile foundation of offshore wind turbines, and the variable diameter section may have an impact on the vertical bearing characteristics of pile foundation. A numerical model of a variable diameter monopile was established by the finite element analysis software ABAQUS, and a series of numerical simulations on the vertical bearing performance of variable diameter monopiles and the influence of their size parameters on the vertical bearing capacity were carried out. The numerical analysis results show that the variable diameter characteristics have little influence on the axial force distribution of the pile compared with the pile with the same diameter. With the increase of vertical load, the proportion of each bearing component of the two types of monopiles remains unchanged at first, and then the proportion of external side friction resistance decreases, while the proportion of internal side friction resistance and pile tip resistance increases. Increasing pile diameter at bottom of variable diameter pile can effectively improve the vertical bearing capacity and increase the proportion of the external side friction resistance. Increasing the upper length and the length of the variable diameter section has no obvious effect on the vertical ultimate bearing capacity of the pile foundation, but will lead to the decrease of the proportion of the external side friction resistance and the increase of the proportion of the internal side friction resistance.