The energy density of non-aqueous carbon-based electrochemical capacitors (cEC) is mainly determined by the specific capacitance and operational voltage range. In this study, we propose to construct an unbalanced structure to make full use of stable voltage range for improving energy density. The stable voltage range is firstly carefully explored using cyclic voltammetry. Then an unbalanced carbon-based electrochemical capacitor (ucEC) is constructed with an optimized positive electrode to negative electrode weight ratio and voltage range. Its electrochemical performance is comprehensively investigated, including energy density, power density as well as cycle life. The ucEC is capable to deliver an improved energy density up to 64.9 Wh/kg (1.4 times as high as a general cEC) without sacrificing the power density and cycle life. The electrode properties after cycling are also analyzed, illustrating the change of electrode potential caused by unbalanced structure. The proposed structure demonstrates a great potential for improving the energy density at little cost of electrode design and cell configuration.