The poly(3′,4′-ethylenedioxy-2,2′:5′,2″-terthiophene)/graphene nanoplatelet (poly(TET)/GNP) composites were synthesized by varying the ratio of GNP from 4wt% to 16wt% through mechanochemical method. The structure and morphology were characterized by the Raman spectroscopy, ultraviolet–visible (UV–vis) absorption spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). The electrochemical performances of the composites were investigated by galvanostatic charge/discharge measurement, cycling stability, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results from Raman and UV–vis spectra showed that the composites were successfully synthesized, and the composites displayed a higher degree of conjugation than the pure poly(TET). The XRD and TEM results proved that the GNP existed in composites, and the ratio of GNP slightly affected the morphology of the poly(TET). The comparison results suggested that the composites showed a higher electrochemical activity than pure poly(TET). Moreover, all composites showed a lower internal resistance and better capacitive behavior than the poly(TET). Due to synergistic effects of GNP and poly(TET), the composite with 12wt% GNP has better electrochemical activity than others. And, the specific capacitance of the composite with 12wt% GNP reached to 206F/g, and had a capacitance retention rate of 78% over 1000 cycles.