The precise control of the pore sizes at an atomic level has proved to be the biggest challenge of all for nanoporous graphene membranes for gas separation. Here, we propose a simple method to realize the selective molecular sieving through originally nonselective graphene nanopores by adding charges on the graphene surfaces. Molecular dynamic simulations show that the CO2/N2 selectivity of the graphene nanopore with a diameter of 0.52 nm increases up to 22.78 for a surface charge density of only -5.934 e/nm2. The selectivity improvement is related to the distinctive adsorption intensities of CO2 and N2 molecules on the charge-loaded graphene surfaces. This work points toward a promising road to tune the selectivity of graphene nanopores and therefore promotes the realization of porous graphene membranes and other two-dimensional porous membranes by accepting the pores with a wide size distribution and reducing the requirements in the control of pore sizes.