Doping and compositing are considered to be the effective strategies for improving the performance of supercapacitor electrode materials. In this work, the Al2O3-doped Co3O4/graphene nanocomposites (denoted as ACGC) were successfully fabricated by a combination method of hydrothermal and calcination. The phase, composition and morphology of fabricated nanocomposites were characterized via using XRD, XPS, TEM, and SEM, and the electrochemical behavior were examined via CV, GCD, EIS. The results showed that the nanocomposites had more specific capacitance (968 F/g) than that of Al2O3-doped Co3O4 nanocomposites (denoted as ACC) (682 F/g) and pure Co3O4 (295 F/g), respectively. Moreover, it had good capacitance retention of 96.9% after 2000 cycles at a high current density of 10 A/g. In addition, an asymmetric supercapacitor (ASC) assembled by ACGC and commercial activated carbon (AC) can provide a specific capacitance of 106 F/g at 0.5 A/g and 84.2% capacitance retention after 5000 cycles at 10 A/g. And it is worth noting that this ASC can provide a maximum energy density of 40.1 Wh/kg and power density of 8285.9 W/kg. Significantly, two ASCs in series can light a red light-emitting diode (LED) (2.2 V) for 200 s and a blue LED (3.2 V).