Carbon nanotubes (CNTs) were modified by hydrothermal treatment by using chlorosulfonic acid and studied as electrocatalyst for V(V)/V(IV) reaction in vanadium redox flow battery. After treatment, CNTs were functionalized with many sulfonic groups, no causing obvious change for the morphology. Contact angle measurements confirm the improvement of wettability of sulfonated CNTs. The introduced sulfonic groups act as active sites and also result in more defects. As a result, the electrocatalytic properties of CNTs toward V(V)/V(IV) reaction are dramatically enhanced by sulfonation treatment. Among sulfonated CNTs, the sample treated for 10hours (CNT-S10) shows the highest electrocatalytic properties. The cell employing CNT-S10 as positive electrocatalyst was assembled, and the performances were assessed. The cell using CNT-S10 exhibits better electrochemical properties, including cycling and rate performance. The energy efficiency of the cell increases by 4.9% by using CNT-S10 electrocatalyst. The excellent electrocatalytic properties of sulfonated CNTs are mainly attributed to the dramatical improvement of hydrophilicity of CNTs and introduction of sulfonic groups as active sites, correspondingly enhancing the mass transfer performance and electrochemical activity toward V(V)/V(IV) reaction. Our study reveals that sulfonated CNTs are a promising and efficient positive electrocatalyst for vanadium redox flow battery.