Herein, we report a facile and efficient method for PL modulation of 1L-MoS2 via photochemical functionalization of graphene in G/MoS2 heterostructures. Monotopic covalent modification of graphene is carried out by a laser-induced reaction between graphene and benzoyl-peroxide (BPO), a phenyl radical precursor. In G/MoS2, the functionalized area of graphene (F-G/MoS2) exhibits a 5-fold enhancement in the PL intensity of MoS2 compared to the non-functionalized area of graphene. Furthermore, the reverse stacked MoS2/F-G heterostructure is fabricated by in situ functionalizing MoS2/G/BPO by photochemical reaction and only shows 1.8 times PL increase compared to pristine G/MoS2. Thus, not only the electron transfer but also interlayer distance plays a certain role in the PL modulation of G/MoS2 heterostructures. Moreover, the PL increase of MoS2 is reversible with defunctionalization of F-G/MoS2 heterostructures by thermal annealing at 400 °C. Thus, F-G is not only highly stable but also plays a flexible role in the PL adjustment of 1L-MoS2.