Abstract: During the operation of a vertical axis wind turbine,the interaction between the upper and lower surface boundary layer and the shear layer causes the formation of periodic vortex structure in the downstream wake of the wind turbine,which has an important effect on the aerodynamic characteristics of the wind turbine. Based on this,the CFD method is used to study the wake vortex structure of vertical axis wind turbine under different working conditions. Fast Fourier transform and phase space trajectory are used to analyze the vortex shedding phenomenon and wake vortex structure of wind turbine blade under different tip speed ratios. The fractal dimension is used to study the torque and wake flow velocity variation. The results show that the wake vortex structure of the wind turbine presents different characteristics with the change of the tip speed ratio. When the tip speed ratio is 3.6,the two sides of the wind turbine wake show regular reverse shedding vortex mode. The wake of the vertical axis wind turbine with low tip ratio has obvious chaotic characteristics,and the chaotic characteristics gradually weaken with the increase of the tip ratio. With the increase of the tip speed ratio,the fractal dimension of wind turbine torque and downstream speed decreases continuously,and when the tip speed ratio is 3.6,the fractal dimension of downstream speed of wind turbine is only 1.07.