Hydrodynamic performance of a fixed U-shaped oscillating water column (U-OWC) wave energy converter is numerically investigated. Based on the time-domain higher-order boundary element method (HOBEM), a two-dimensional fully nonlinear numerical model is implemented to simulate the nonlinear wave interaction with a U-OWC device. In the model, the inner-domain-source method is adopted to generate incident waves and a linear pneumatic model is used to determine the air pressure which is imposed on the free surface inside the chamber. The numerical model is well validated against the published experimental data of the free surface elevation at the chamber centre, air pressure inside the chamber and hydrodynamic efficiency. Further, the present model is applied to study the effects of geometrical parameters (including the vertical duct height, vertical duct width and wall thickness) on the hydrodynamic performances of the U-OWC device. The results indicate that geometrical parameters of the vertical duct have significant influence on the air pressure inside the chamber and the hydrodynamic efficiency. The hydrodynamic efficiency and air pressure inside the chamber are found to increase with the increase of both vertical duct height and thickness of wall I. However, the increasing rate of the efficiency slows down gradually with the thickness of wall I increasing, which indicates that a comprehensive consideration of the construction cost and hydrodynamic performance is needed in the design and construction of a U-OWC device.