In the wave energy converter, the performance of buoy motion can affect the energy absorption of the device directly. A three-dimensional numerical wave tank was built, which was based on CFD method using N-S equation. By simulating the time-domain motion of buoy and comparing with the experimental results, the feasibility of the CFD method was verified. Based on this, the effect of buoy motion on the surrounding fluid was analyzed using the flow field and the energy absorption regular of single and double DOFs was studied. It is shown that the buoy makes heave or pitch motion, the capture width ratio appears the maximum value with the increase of the PTO damping coefficient, the capture width ratio has the best PTO damping, the capture width ratio of double DOF buoy is higher than that of the single DOF buoy, the two DOF can affect each other in the energy absorption by comparing the different DOFs of the buoy.