Co3O4 porous nanosheets were fabricated through two-step synthetic method: water bath stir process followed by annealing treatment. As an electrode for battery, the achieved sample exhibits excellent electrochemical performance. The cyclic voltammetry (CV) curve shows that the sample possesses two pairs of redox peaks, revealing good redox behavior of Co3O4. Both the two kinds of valence (Co2+ and Co3+) participated in Faradic reaction. The Co3O4 porous nanosheet reveals a high specific capacity of 583.1 C g−1 at a scan rate of 1 A g−1. With the obtained sample as positive electrode and commercial activated carbon (AC) as negative electrode, the Co3O4//AC asymmetric cell is able to achieve high voltage window of 0–1.7 V. The cycle capability of the as-made asymmetric device possesses 83.5% retention after 10,000 galvanostatic charge/discharge cycles. The energy density varies from 33 Wh kg−1 to 21 Wh kg−1 at a power density ranging from 798 W kg−1 to 18,900 W kg−1.