In this paper, sulfur and nitrogen co-doped graphene quantum dots (S,N-GQDs) were compounded with titanium dioxide (TiO2) nanorods by chemical surface modification. S,N-GQDs in diameter of 5 nm are uniformly distributed on the surface of TiO2 nanorod arrays. The presence of CS bonds in S,N-GQDs not only broaden the absorption of visible light by GQDs, but also enhanced the visible light absorption of S,N-GQDs/TiO2 composites. The photocurrent densities of S,N-GQDs/TiO2 were 3.66 times that of unmodified TiO2. The calculation results of density functional theory (DFT) showed that the band gap of S,N-GQDs is significantly reduced compared to GQDs, indicating that electrons are more likely to transit to higher energy levels and form more activation sites. Moreover, compared with TiO2, the DFT calculation results of band gap, density of states and charge density difference of S,N-GQDs/TiO2 showed that S,N-GQDs/TiO2 had a wide wavelength of visible light absorption and better electron-hole separation ability, resulting in the superior photoelectrocatalytic property. This research provides favorable analytical value for the development of photocatalysts based on heterostructures. S,N-GQDs/TiO2 nanocomposites with excellent photocatalytic activity were prepared by simple surface modification. N–H, C–N, CS functional groups of S,N doped GQDs is conducive to the absorption of visible light and the enhancement of catalytic activity. Both theoretical and experimental results confirmed the extension of optical absorption into the visible range and the frequent charge transfer from S,N-GQDs to the TiO2 surface, facilitating electron-hole separation and reducing charge recombination rate. Display omitted •S,N-GQDs/TiO2 prepared by chemical surface modification present good electron-hole separation ability.•S,N-GQDs broaden the visible light absorption range and promote the separation of electron-hole pairs of TiO2.•The band gap, density of states and charge differential density of nanocomposites were studied by DFT.