Abstract: Floating breakwaters are usually moored by lines or restrained by piles. The two mooring systems have different performances in hydrodynamics, motion response and wave dissipation. Based on the Navier-Stokes equation, this paper presents a 2-dimentional numerical model to study the hydrodynamic characteristics of pile-restrained floating breakwaters under regular waves. The model utilizes the Constrained Interpolation Profile (CIP) scheme to solve the convection term in the control equation, and multi-phase flow and Immersed Boundary (IB) methods to deal with the fluid-structure interaction. The present model is applied to analyze the influence of the relative breakwater width on the hydrodynamic characteristics including heave-motion response, wave forces and moments. Partial results are similar to the existing experimental results, indicating that this numerical model can simulate the motion and force responses of pile-restrained floating breakwaters.