Display omitted •Activated carbon layer covered graphene (ACG) with the adjustable thickness has been fabricated.•The electrochemical performance of ACG is related to the thickness of the active layer on the surface of the ACG.•The ACG with a suitable thickness of 1–2nm exhibits a high specific capacitance of 248.4Fg−1 at 1Ag−1.•An outstanding rate capability has been obtained with a capacitance of 187.5Fg−1 at 100Ag−1. Carbon-based nanostructures have drawn special attention for use in supercapacitive applications due to their high specific surface area and stability. Balancing the conflict between activity and the conductivity of carbon based electrode materials would contribute to comprehensive energy storage performance for practical application. In this work, an activated carbon layer covered graphene (ACG) with adjustable thickness has been fabricated by hydrothermal treatment of graphene oxide with glucose and further activation with a KOH agent. The electrochemical performance of ACG is related to the thickness of the active layer on the surface of the ACG. The ACG with a suitable thickness of 1–2nm exhibits a high specific capacitance of 248.4Fg−1 at 1Ag−1 and perfect rate performance with a high capacitance of 187.5Fg−1 at a large current density of 100Ag−1 owing to both high activity and conductivity. Particularly, the synergetic effect of the active porous carbon and conductive graphene results in considerable energy properties of the ACG based symmetric devices.