The enhancement of aerodynamic performance of airfoil using Gurney flap was demonstrated and due to its effects of lift increasing and stall suppression on airfoil, Gurney flap has positively functioned in a wide range of engineering. Though there had been numerous scholars who aroused wide concern on geometric parameters of Gurney flap for airfoil, the investigation of geometric design of Gurney flap for straight-bladed vertical axis wind turbine is absent. In current research, a comprehensive numerical study on the effect of geometric parameters of Gurney flap on performance enhancement of straight bladed vertical axis wind turbine. Before a validation between numerical results and experimental data was carried out, a novel mathematical model of resistance torque of struts was put forward to increase the reliability of numerical model. The results show that the Gurney flap can remarkably promote the aerodynamic performance of vertical axis wind turbine with a decreased rotational velocity. The maximum improvement can reach up to 21.32%. In upstream area, Gurney flap can considerably increase the blade tangential force. Short-Gurney-flap blade can effective weaken the deficiency of aerodynamic loss in downstream area. The 0.75%-chord-length height is most appropriate for the straight-bladed vertical axis wind turbine in this paper. The aerodynamic load with Gurney flap is not sensitive to width. The design with 0.12%-chord-length width is the most suitable value through analyzing the power coefficient curve. (C) 2020 Elsevier Ltd. All rights reserved.