G-C₃N₄, a biocompatible material, has prominent applications in biology and is ideal for nano-enzyme studies. Though reported as a peroxidase mimic, its activity remains low. This group combined N,S-doped carbon quantum dots (NS-CQDs) with g-C₃N₄ (7NSC-g), verifying its peroxidase-like activity. Based on this, a ternary composite of Co₃O₄ in different forms and 7NSC-g was developed to enhance peroxidase activity, to design a g-C₃N₄-based composite enzyme. Characterizations determined the composition and morphology. Colorimetry evaluated peroxidase activity, where the simulated enzyme catalyzes blue product formation from the TMB substrate in the presence of H₂O₂. UV-Vis spectrophotometry measured absorbance changes to determine target concentrations. The results show Co₃O₄ doping improves catalytic activity, with larger specific surface area providing more activation sites. The highest activity of g-C₃N₄/NS-CQDs/Co₃O₄ was at 5% floral Co₃O₄, being efficient due to Co₃O₄’s electron-transfer acceleration and hydroxyl-radical mechanism. Under optimal conditions, the composite detected H₂O₂ (10.0–230.0 μM, detection limit of 0.031 μM) and glucose (10.0–650.0 μM, detection limit of 1.024 μM).