In this paper, carbon microsphere prepared by hydrothermal treatment was activated by KHCO3 at high temperature, and employed as the catalyst for VO2+/VO2 + redox reaction for vanadium redox flow battery (VRFB). Carbon microsphere can be etched by KHCO3 due to the reaction between the pyrolysis products of KHCO3 and carbon atoms. Moreover, KHCO3 activation can bring many oxygen functional groups on carbon microsphere, further improving the wettability of catalyst and increasing the active sites. The electrocatalytic properties of carbon microsphere from hydrothermal treatment are improved by high temperature carbonization, and can further be enhanced by KHCO3 activation. Among carbon microsphere samples, the VO2+/VO2 + redox reaction exhibits the highest electrochemical kinetics on KHCO3 activated sample. The cell using KHCO3 activated carbon microsphere as the positive catalyst demonstrates higher energy efficiency and larger discharge capacity, especially at high current density. The results reveal that KHCO3 activated carbon microsphere is an efficient, low-cost carbon-based catalyst for VO2+/VO2 + redox reaction for VRFB system.