The effects of magnetic fields on electrochemical processes have made a great impact on both theoretical and practical significances in improving capacitor performance. In this study, active carbon/Fe3O4-NPs nanocomposites (AC/Fe3O4-NPs) were synthesized using a facile hydrothermal method and ultrasonic technique. Transmission electron micrographs (TEM) showed that Fe3O4 nanoparticles (Fe3O4-NPs) grew along the edge of AC. AC/Fe3O4-NPs nanocomposites were further used as an electrochemical electrode, and its electrochemical performance was tested under magnetization and non-magnetization conditions, respectively, in a three-electrode electrochemical device. Micro-magnetic field could improve the electric double-layer capacitance, reduce the charge transfer resistance, and enhance the discharge performance. The capacitance enhancement of magnetized electrode was increased by 33.1% at the current density of 1 A/g, and the energy density was improved to 15.97 Wh/kg, due to the addition of magnetic particles.