Tidal current power generation is attracting more and more attention these days for its cleanliness, predictability and reliability.A brimmed diffuser from wind-lens technology is tested in this study in order to observe its effect on power generation efficiency in tidal current turbine systems. Validation experiments were carried out in the circulating water channel for the bare turbine and the shrouded turbine, results of which proved that the diffuser improved the performance of the tidal turbine. The computational fluid dynamics (CFD) method and the blade element momentum (BEM) theory were used to evaluate the performance of the bare turbine and the shrouded turbine. For tip speed ratios (TSRs) of 2.5 to 4.0, both CFD and BEM calculations for the bare turbine were well-correlated with the experimental data. For the shrouded turbine, CFD results failed to exactly reproduce the experimental results with the turbulence model and mesh size utilized, but BEM results did succeed with TSRs greater than 3.0. Considering correlation with experimental data and calculation time cost, BEM is a satisfactory method for evaluating the performance of a tidal current turbine.