Abstract: This study investigates the hydrodynamic characteristics of a submerged horizontal circular cylinder under forced oscillation by using a 2-D two-phase flow numerical wave tank (NWT) model based on the viscous fluid theory. The calculations of the hydrodynamic forces on the circular cylinder under various liquid-phase viscosities were performed. By studying the relationship between the hydrodynamic forces and motion of the cylinder and analysing the flow fields around the cylinder, the mechanism of the viscous effects was identified. The study shows that differences between the results of the cases under different viscosities are on account of different contributions of the viscous shear forces to the total hydrodynamic forces and different influences of the vortex on the hydrodynamic forces. The considerable influences of fluid viscosity on the motion hysteresis of the vortex lead to a phase difference between the pressure forces on the cylinder under different viscosities. The phase advance phenomenon of the viscous shear forces can be explained by the fact that compared with the motion of the cylinder, the motion hysteresis of the water-particles in the higher viscosity fluid is more obvious.