The performance of a hydrokinetic turbine may be significantly overestimated in a confined domain due to blockage effects. Therefore, the performance data needs to be corrected to obtain the unconfined results. Although many studies on blockage correction are performed, most of them are only developed for conventional hydrokinetic turbines. And the effectiveness of these methods for a diffuser-augmented hydrokinetic turbine is still unclear. For this investigation, seven different blockage correction methods are evaluated for a diffuser-augmented hydrokinetic turbine. A blade-resolved Computational-Fluid-Dynamics simulations is performed and then verified with the experiment data. The results show that the methods presented by Barnsley and Wellicome, Houlsby et al. and Maskell-inspired's method have the best performance based on both the relative errors analysis and the ease of application of each method.