In this paper, a novel self-rectifying impulse turbine with U-shaped duct used for oscillating water column wave energy conversion is proposed. It has a pair of fixed, symmetrical and radial-flow guide vane rows, and an axisymmetric, U-shaped duct with increasing cross-sectional area from the rotor to the guide vane rows, therefore it is expected to be relatively simple, reliable and efficient. A three-dimensional (3D) steady-state numerical model and a fully transient model are established for the present turbine. These two models are validated by comparison with experimental results from the published works, respectively, and then used to investigate the steady performance and transient characteristics of the impulse turbine. Effect of the geometry of the guide vanes, the number of the guide vanes, and the hub-to-tip ratio on turbine performance under steady airflows and transient airflows are studied, and the difference between these two models in predicting turbine performance is discussed.