Taking an eccentric cylindrical annulus as an example, a CFD-based method for predicting added mass and fluid damping was established, and the effects of eccentricity and amplitude on the results were compared to provide key inputs for the structural seismic and fluid induced vibration analyses. In this paper, the numerical simulation of the flow field be realized by User Defined Functions and overset mesh technology. The shape of the function was determined according to Bearman's hypothesis, and the calculated fluid force curve was fitted by the least square method to obtain the added mass coefficient and fluid damping coefficient. The calculation and analysis results show that (1) the calculated results are very close to the literature results, which illustrate the effectiveness of this method and compensates for the inability of the potential flow theory. (2) The added mass increases with the eccentricity. (3) At values of amplitude less than a certain value, the effect on the added mass is negligible, and above this value, the added mass decreases rapidly. (4) The fluid damping increases with the increase of eccentricity and amplitude. By decomposition of fluid damping, it is found that the ratio between pressure damping and viscous damping is kept in a relatively constant range, and the viscous damping is dominant. Therefore, the effect of amplitude on the added mass can be neglected for most critical components in primary coolant circuit, the effect of eccentricity and special-shaped structures must be taken into account. The established methodology enables the consideration of this effect.