When navigating near surface，underwater vehicle will cause inertial motion of the fluid in the surrounding flow field，and the kinetic energy，meanwhile，will dissipate with the wave diffusion. The natural period and amplitude attenuation velocity of the vehicle during near-surface movement are affected by the water，and can be described by added mass and damping coefficient. In this paper，the computational fluid dynamics software STAR-CCM+ is used to carry out the numerical simulation of free roll and pitch attenuation motions of the near-surface vehicle. According to the simulated free attenuation time history curves obtained under different initial motion amplitudes，the added mass and damping coefficient of roll and pitch can be calculated respectively. Combined with the actual motion，the change causes for the added mass and damping coefficient under different initial motion amplitudes are analyzed respectively.
