Graphene has been widely used for electrical energy storage and its performances could be further improved by heteroatom doping. How to prepare doped graphene efficiently and economically remains a significant challenge. Here, we propose a flash-assisted doping method to produce nitrogen- and sulfur-doped graphene (N-rGO and S-rGO). Using this method, graphene oxide (GO) is reduced to few-layer graphene (rGO) in seconds without the use of reductants, accompanied with a high doping efficiency. Particularly, the as-synthesized N-rGO with a high N content of 12.75 at.% used as potassium-ion battery (KIB) anode exhibits ultrafast K⁺-transport kinetics and superior K⁺-storage capability. Quantitative kinetics analysis and theoretical simulation are used to reveal the mechanism of transportation and storage of K⁺ in N-rGO.