Abstract: This study investigated the wave forces on an incompletely submerged horizontal circular cylinder under large-amplitude wave action using a viscous fluid numerical wave tank model based on the finite volume method. By analysing the characteristics of the wave and slamming forces on the circular cylinder,the influences of wave amplitude and vertical position of the cylinder on the wave forces were investigated,and the relationships between the slamming forces and velocity were discussed. The slamming coefficients were based on a data fitting method. The results showed that the wave forces were significantly influenced by wave amplitude and vertical position of the cylinder. The mechanism can be explained by the features of the fluid motion in the wave field. Although the slamming forces were closely related to the slamming velocity,the influence of the vertical position of the cylinder on the minimum predicted slamming velocity for the situation,in which the slamming forces play a dominant role in the wave forces,were unremarkable. As the wave amplitude increases,the distribution of the slamming coefficients for different vertical positions of the cylinder changes from dispersed towards focused.