MoS₂ quantum dots (QDs) functionalized g-C₃N₄ nanosheets (MoS₂@CNNS) were prepared through a protonation-assisted ion exchange method, which were developed as a highly efficient biomimetic catalyst. Structural analysis revealed that uniformly-dispersed MoS₂ QDs with controllable size and different loading amount grew in-situ on the surface of CNNS, forming close-contact MoS₂@CNNS nanostructures and exhibiting distinct surface properties. Compared to MoS₂ QDs and CNNS, the MoS₂@CNNS nanocomposites exhibited a more than four times stronger peroxidase-like catalytic activity, which could catalyze the oxidation of 3,3’,5,5’-tetramethylbenzidine (TMB) in the presence of H₂O₂ to generate a blue oxide. Among the MoS₂@CNNS nanocomposites, MoS₂@CNNS(30) was verified to present the best intrinsic peroxidase-like performance, which could be attributed to the more negative potential and larger specific surface area. A simple, rapid and ultrasensitive system for colorimetric detection of H₂O₂ was thus successfully established based on MoS₂@CNNS, displaying nice selectivity, reusability, and stability. The detection limit of H₂O₂ could reach as low as 0.02 μM. Furthermore, the kinetic and active species trapping experiments indicated the peroxidase-like catalytic mechanism of MoS₂@CNNS. This work develops a novel, rapid, and ultrasensitive approach for visual assay of H₂O₂, which has a potential application prospect on clinical diagnosis and biomedical analysis.