Plant microbial fuel cell (PMFC) newly emerges as a sustainable technology in green infrastructures such as green roofs, reactive barriers, and environmental biosensors. PMFCs generate bioelectricity driven by microbial redox reactions in the rhizosphere. However, the interactions between the bioelectricity and soil–water characteristics remain unknown, and PMFCs are vulnerable to drought stress. Therefore, this study aims to reveal the coupled bio-hydrological behaviors of PMFCs and propose biochar amendment on PMFCs for improving bioelectricity generation under drought conditions. Biochar-aided PMFCs were prepared by growing green-roof vegetation, Hydrocotyle vulgaris, on sandy lean clay with biochar at a mass ratio of 0, 5, and 10%. Bioelectricity (i.e., electrical potential and current) and unsaturated soil properties (i.e., suction and water content) were monitored simultaneously. Results showed that biochar increased the electrical power output of PMFCs by up to 30 times in unsaturated conditions. This may be because biochar-enhanced catalytic activities in redox reactions decreased soil electrical resistance and improved water retention. The bioelectricity was highly correlated with the suction, as the bioelectrical potential and current decreased by 90% with increasing suction. It was also found that the bioelectrical potential dropped rapidly by 67.6–87.2% when suctions were larger than the air entry values. This might be because air invasion led to the discontinuity of the liquid phase, hindering the ion transport of microbial redox reactions. The study implies that bioelectricity could be applied for field monitoring of soil–water characteristics, and biochar addition would enhance the drought resistance of PMFCs.