As a carbon resource with abundant nitrogen, polyacrylonitrile (PAN) has been used for the key raw materials to produce carbon materials. However, the direct carbonization will lead to the cementation of PAN particles, which is adverse to the subsequent activation. In this work, a hybrid porous carbon (HPC) was synthesized via dry ball-milling of thermal-reduced graphene and polyacrylonitrile, and subsequent stabilization and KOH activation. The effect of the ratio of graphene and polyacrylonitrile, along with activation treatment on the properties of hybrid porous carbon are systematically studied. The results demonstrate that the existence of graphene nanosheets enable the fast dissipation of heat produced in ball-milling process and avoid the cementation of PAN particles, while PAN particles act as spacers to prevent graphene restacking. The as-obtained polyacrylonitrile/graphene precursor is a loose powder, which favors the dispersion of activation agents within the precursor and results in a more homogeneous and effective activation. Meanwhile, graphene works as a 3D micro-current collector, thus providing a conductive network for convenient charge transfer in HPC. Based on the electrochemical characterization in either water or a non-aqueous electrolyte, HPC exhibits a superior capacitive behavior because of its well-developed porosity, large specific surface area, excellent electrical conductivity as well as nitrogen/oxygen heteroatom induced pseudo-capacitance. Particularly, HPC-4 offers a high energy density of 30.38 W?偸h/kg at a power density of 337.5 W/kg in TEABF4/EC-DMC electrolyte. This work provides an easy-to-operate and efficient synthesis strategy for developing porous carbon electrode towards supercapacitors with high power and energy density.