The use of transition metal oxynitrides as electrode has low cost and high energy density, providing asymmetric supercapacitor devices with an ideal power source. However, the mechanism of inducting foreign nitrogen atoms into transition metal oxides to improve electrochemical performance is unclear. In this work, a transition metal oxynitride layer on flexible carbon cloth (CoON-CC) is presented for the structural design of electrode, which creatively develops an excellent conductivity, numerous active sites and outstanding stability comprising Co3O4 nanorods on flexible carbon cloth electrode (Co3O4-CC). For the first time, a flexible asymmetric supercapacitor (FAS) is prepared with CoON-CC and active carbon electrode (AC-CC) as the positive and negative electrodes, respectively, delivers a high aeral capacitor of and 2.12 F cm−2 (651.5 F g−1) and energy density of 1.17 Wh cm−2 as well as excellent flexibility and cycle performance, outperforming previously reported Co-based asymmetric supercapacitor. Experimental research results and density functional theory calculations reveal that the foreign nitrogen atoms can improve the energetic for redox reaction by modulating the free energy for adsorbed intermediates (OH−, O∗, OOH∗) and also enhance the conductivity, further improving the electrochemical performances of the FAS.