Floating vertical axis tidal current turbine (VATT) will occur pitching motion response in waves. To study the effect of pitching motion, a complex model of VATT moving in the waves is simplified to an impeller moving in the uniform current and superposed with harmonic motions, and the hydrodynamic performance of VATT is studied based on CFD viscous theory and ANSYS-CFX software, as well as the moving mesh and the sliding mesh. The CFX software is used to simulate the pitching motion under different speed ratio and rotating impeller is forced to pitch. The thrust coefficient and lateral force coefficient time history curves are also analyzed under different speed ratio by the method of least squares. The results show that, compared with turbine only rotating in uniform stream, the turbine under pitching motions has a little higher value in turbine's energy utilization. The instantaneous fluctuation amplitude of the thrust and lateral force under pitching motion are obviously higher than that of turbine under uniform stream, which is bad for the structural strength and fatigue effect of turbine in waves. The damping force is the main ingredient to turbine under pitching motion and the effect of added mass force is small.