The addition of H2O, even trace amount, in aprotic Li-O2 batteries has a remarkable impact on achieving high capacity by triggering solution mechanism, and even reducing charge overpotential. However, the critical role of H2O in promoting solution mechanism still lacks persuasive spectroscopic evidence, moreover, the origin of low polarization remains incompletely understood. Herein, by in situ spectroscopic identification of reaction intermediates, we directly verify that H2O additive is able to alter oxygen reduction reaction (ORR) pathway subjected to solution-mediated growth mechanism of Li2O2. In addition, ingress of H2O also induces to form partial LiOH, resulting in reduced charging polarization due to its higher conductivity; however, LiOH could not contribute to O2 evolution upon recharge. These original results unveil the complex effects of H2O on cycling the aprotic Li-O2 batteries, which are instructive for the mechanism study of aprotic Li-O2 batteries with protic additives or soluble catalysts.