Producing electricity from renewable energy does not necessarily mean cleanness and sustainability, hence the associated with environmental impacts should be paid attention to and thoroughly investigated. This study presents an LCA of the prototype buoy-rope-drum (BRD) WEC, and systematic uncertainty and sensitivity analyses, to increase understanding of the uncertainties and sensitivities of the life cycle impact variations to the inputs, method choices, as well as changes of selected key issues. The IMPACT 2002+ method is used for a full LCA, including mid-point, end-point and single-score impact assessments. The Monte Carlo (MC) method is applied for the uncertainty analysis for the propagating effects of input variations on the output life cycle impact assessment (LCIA) results. Beyond scope of IMPACT 2002+, we investigated the uncertainties due to variations of maritime climate and interactions between the device and surrounding ecosystems. Analytical and scenario analyses are adopted to conduct the sensitivity analyses. The uncertainty analysis shows that our LCIA results have reasonable uncertainties to the input variations. Ionizing radiation and human health are found to have the largest 95% confidence intervals among mid-point and end-point impacts. The results also suggest that future installation location associated with uncertainties from in-situ maritime climate, interactions with surrounding ecosystems, etc., should be investigated. The results of the sensitivity analysis show that the maximum sensitivity is the variation of Ozone layer depletion due to 20% increment of buoy steel weight. Furthermore, the choices of impact assessment methods are found to have influence to the results, mainly due to differences of embedded time horizons in these methods. Carefully dealing with the quality of input data and choices of appropriate methods will help reduce the instability and improve the level of overall reliability of the LCIA results.