The rapid development of air turbines for oscillating-water-column wave energy conversion proposes a crucial demand for deep understanding of its unsteady performance in the reciprocating air flow condition. A 3D unsteady numerical model is presented in this study, which is established on the platform of commercial computational fluid dynamics (CFD) software ANSYS-Fluent 12.0. The passive rotation of turbine rotor is fully driven by the bi-directional air-flows following the Newton's second law. The numerical model is validated by experimental results within the grid independency and time-step sensitivity studies. The 3D overall pneumatic characteristics in a stable wave cycle are presented to demonstrate the unsteady performance of the turbine rotor. Effects of air-flow velocity amplitude and rotor's moment of inertia on the flow field and pressure distribution on the blades, pneumatic torque and rotation speed, and the transient input and torque coefficients are studied. (C) 2017 Elsevier Ltd. All rights reserved.