In order to study the three-dimensional effect on the oscillated vertical axis tidal turbine, the CFX software was employed to simulate the oscillated (surge and sway) turbine with different span-chord ratios. The simulation results were then compared with those in 2D situation. And then, the time series curves of thrust and lateral force coefficients of the hydraulic turbine with different aspect ratios were fitted by least square method, and the damping and additional mass coefficients were obtained. The results show that the current power output efficiency of the turbine increases with the span-chord ratio increased. The hydrodynamic load of three-dimensional oscillating tidal turbine is positively correlated with the elongation of the turbine. When the spanchord ratio reaches 10, the hydrodynamic loads and power conversion efficiency of the 3D oscillating turbine are basically the same as those of the 2D one. The constant and damping terms of the hydrodynamic coefficients of the 3D turbine increase with the increase of spanchord ratio.