This paper investigates the effects of optimized airfoil on VAWT (vertical axis wind turbine) aerodynamic performance. The thickness and camber of the airfoil are selected as the constraints, the value of the maximum tangential force coefficient is chosen as the objective function, optimizing NACA0015 airfoil to enhance the wind energy utilization efficiency of the VAWT, a 3D CFD simulation is used to get the flow characteristics of the VAWT under variable tip speed ratio (TSR) conditions. To ensure the accuracy of the numerical simulation, the power coefficient calculated by CFD is validated against previous experimental result. The optimized airfoil is shown to improve the aerodynamic performance of the wind turbine. Through investigating the effects of optimized airfoil on the rotor flow field, this paper proposes measures to improve the VAWT aerodynamic performance under variable TSRs: measures should be made to avoid or delay the flow separation of blades and the stall vortex shedding and reduce the stall vortex scale at low TSR, while at high TSR measures should be made to shorten the wake length of the blades and reduce its diffusion range.