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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.