Graphitic carbon nitride (g-C3N4), as a metal-free two-dimensional nanomaterial, has a broad prospect for photocatalysis. In this paper, alkali-exfoliated g-C3N4 has been modified by sodium tert-butoxide from bulk g-C3N4, where a family of composite materials has been prepared by tuning the dosage of graphene quantum dot (GQD). Simultaneously, the morphology, structure, and composition of these samples have been fully confirmed by XRD, FT-IR, SEM, UV-Vis, PL, and other characterization methods, as well as the photodegradation of methylene blue. The results indicate that compared with bulk g-C3N4, alkali-exfoliated g-C3N4 can shorten the carrier transport distance, and the introduction of GQD can effectively improve the recombination of electron-hole pairs. When the doping amount of GQD is 5.0%, the photocatalytic degradation efficiency of methylene blue reaches 90.9%. In addition, the active groups of photodegradation have been identified as?O2- and h+ by free radical trapping experiments, and a possible methylene blue light catalytic mechanism is proposed.