When offshore floating vertical axis wind turbines (OF-VAWTs) face the ocean waves and wind loads under normal operation conditions, they have six-degrees of freedom (6-DOF) movement. Each of the 6-DOF movements will influence the aerodynamic performance of the OF-VAWTs in turn. In view of this, the present paper uses the computational fluid dynamics (CFD) method and the Improved Delayed Detached Eddy Simulation (IDDES) to investigate the aerodynamics of an OF-VAWT in periodic surge motion. The overset mesh technique is employed to simulate the rotor's surge motion. In order to verify the present CFD model, the power coefficients of a bottom-fixed VAWT at different tip speed ratios are compared between the experiments and the simulations. By contrast with the non-surge motion, the aerodynamic forces (torque, tangential force, normal force and pressure) and vortex structures of an OF-VAWT are analyzed. Subsequently, the unsteady aerodynamic performance of an OF-VAWT in different amplitudes and periods of surge motion is investigated. It is shown that the surge motion can widen the variation ranges of the aerodynamics forces, and change the flow field around the rotor. The smaller surging amplitude and larger surging period are proposed as they can reduce the variation ranges of the aerodynamics forces, and then keep the floating wind turbines more steady. In addition, the durability and power output of the wind turbines will be improved in surge motion with smaller amplitude and larger period. (C) 2017 Elsevier Ltd. All rights reserved.