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Graphitic carbon nitride (g-C3N4) has been widely investigated and applied in photocatalysis and catalysis, but its performance is still unsatisfactory. Here, we demonstrated that K-doped g-C3N4 with a unique electronic structure possessed highly enhanced visible-light photocatalytic performance for NO removal, which was superior to Na-doped g-C3N4. DFT calculations revealed that K or Na doping can narrow the bandgap of g-C3N4. K atoms, intercalated into the g-C3N4 interlayer via bridging the layers, could decrease the electronic localization and extend the π conjugated system, whereas Na atoms tended to be doped into the CN planes and increased the in-planar electron density. On the basis of theoretical calculation results, we synthesized K-doped g-C3N4 and Na-doped g-C3N4 by a facile thermal polymerization method. Consistent with the theoretical prediction, it was found that K was intercalated into the space between the g-C3N4 layers. The K-intercalated g-C3N4 sample showed increased visible-light absorption, efficient separation of charge carriers, and strong oxidation capability, benefiting from the narrowed band gap, extended π conjugated systems, and positive-shifted valence band position, respectively. Despite that the Na-doped g-C3N4 exhibited narrowed bandgap, the high recombination rate of carriers resulted in the reduced photocatalytic performance. Our discovery provides a promising route to manipulate the photocatalytic activity simply by introducing K atoms in the interlayer and gains a deep understanding of doping chemistry with congeners. The present work could provide new insights into the mechanistic understanding and the design of electronically optimized layered photocatalysts for enhanced solar energy conversion.