Abstract: Vertically aligned carbon nanotubes (VACNTs) were prepared by water-assisted chemical vapor deposition method. The manganese precursor was loaded on VACNTs surface by supercritical carbon dioxide assisted impregnation, and then the Mn₃O₄/VACNTs composite electrode material was prepared by vacuum annealing at different temperatures (250-350 ℃). The microstructure and electrochemical performance of the composite electrode material were studied. The results show that in the composite electrode material Mn₃O₄ nanoparticles were loaded on the surface of carbon nanotubes. After vacuum annealing at 300 ℃, Mn₃O₄ nanoparticles with diameter of 6-10 nm were evenly distributed on the surface of carbon nanotubes. Compared with pure VACNTs, specific capacitance of the composite electrode material increased by 3-4 times, and Warburg impedance, equivalent series resistance and charge transfer resistance at the electrode/electrolyte interface were relatively small. The composite electrode material by vacuum annealing at 300 ℃had the best electrochemical performance; the maximum specific capacitance was 168 F·g^-1, and the specific capacitance retention was 56% with the charge and discharge current density increasing from 1 A·g^-1 to 10 A·g^-1; the equivalent series resistance was the smallest (about 2.5 Ω).