Abstract: In this paper, the boundary layer separation characteristics of straight blade vertical axis wind turbine (H-VAWT) blades under different Reynolds numbers were studied by computational fluid dynamics (CFD) method, aimed to reveal the effect of changes in Reynolds number on the boundary layer separation of blades. Taking the H-VAWT of NACA0018 blade as an object, three-dimensional numerical simulation was carried out by using the SST k-ω turbulence model, and the influence mechanism of different Reynolds numbers on the boundary layer separation characteristics of the blade was analyzed. It is found that the smaller the Reynolds number is, the earlier the boundary layer separation is, the separation point moves forward and the attachment point moves backward. In a rotation period of H-VAWT, blade separation is the most obvious at phase angle 130°. When Reynolds number decreases from 1.6×10⁵ to 0.7×10⁵, the separation point on the upper surface moves forward from x/c=0.125 to x/c=0.017, and the attachment point on the lower surface moves backward from x/c=0.25 to x/c=0.6. The results show that Reynolds number is an important factor affecting the separation of H-VAWT blade boundary layer, and the separation of H-VAWT blade boundary layer is likely to occur at low Reynolds number. In addition, the separation degree of boundary layer is also different at different positions of blade extension. The closer the blade tip is, the smaller the separation degree is, which provides a new perspective for three-dimensional aerodynamic design of H-VAWT.