Studies demonstrate that the reduced frequency k is influenced by the incoming wind speed U₀ and the rotor speed n. As a dimensionless parameter, k characterizes the stability of the flow field, which is a critical factor affecting the performance of vertical-axis wind turbines (VAWTs). This paper investigates the impact of k on the performance of straight-blade vertical-axis wind turbines (H-VAWT). The findings indicate that 0.05 is the critical value of k. The same k results in a similar flow field structure, yet the performance changes vary with different U₀. A decrease in n or an increase in U₀ leads to an increase in the average value and fluctuation of k, which subsequently reduces the rotor rotation torque C_m and decreases the maximum wind energy utilization rate Cp_max. This reduction in Cp_max weakens the stability of the flow field. Additionally, the high-speed area of the blade’s trailing edge velocity trajectory at $\theta =0°$, $\theta =120°$, and $\theta =240°$ expands with increasing range. Velocity dissipation in the high-speed area of the trailing edge affects the stability of the flow field within the rotor.