To improve the specific capacitance and energy density of electrochemical capacitor, nanostructured NiO was prepared by high temperature solid-state method as electrode material. The crystal structure and morphology of as-parepared NiO samples were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Cyclic voltammetry (CV) measurement was applied to investigate the specific capacitance of the NiO electrode. Furthermore, a novel mixed electrolyte consisting of NaOH, KOH, LiOH and Li2CO3 was prepared for the NiO capacitor, and the component and concentration of the four different electrolytes was examined by orthogonal test. The results showed that the NiO sample has cubic structure with nano-size particles, and the optimal composition of the electrolyte was: NaOH 2 mol L^(-1), KOH 3 mol L^(-1), LiOH 0.05 mol L^(-1), and Li2CO3 0.05 mol L^(-1). At a scan rate of 10 mV s^(-1), the fabricated capacitor exhibits excellent electrochemical capacitive performance, while the specific capacitance and the energy density were 239 F g^(-1) and 85 Wh kg^(-1), which was higher than one-component electrolyte.