The convenient detection of organophosphorus pesticides like dimethoate generally depends on the inhibitory effect of natural enzymes, which is especially susceptible to the surrounding conditions and will lead to some unreliable results. This paper reports a colorimetric chemosensor for the simple and rapid detection of dimethoate pesticide in agricultural products. The chemosensor mainly relies on the inhibition of the peroxidase-mimicking catalytic activity of gold nanoparticles (AuNPs). The nanoparticles coupled with hydrogen peroxide can oxidize substrate o-phenylenediamine (OPD) to generate the final product of 2,3-diaminophenazine, which has a characteristic absorption peak at 450 nm and displays a yellow color. In the presence of dimethoate, the catalytic activity of AuNPs is strongly inhibited, resulting in the appearance of light yellow or even colorless solutions. To improve the performance of the current chemosensor, several important parameters such as pH, and the concentrations of substrates and AuNPs have been investigated. Under the optimal catalytic conditions, the catalytic kinetics of peroxidase-mimicking AuNPs is extensively investigated, and it is found that the catalytic reaction conforms to the “ping pong” mechanism. The addition of dimethoate can lead to the decrease in K_m and V_max values of the mimetic enzymes, suggesting that the peroxidase-like activity of AuNPs will be inhibited in an anticompetitive manner. The proposed chemosensor can detect dimethoate in a linear range from 10 μg L⁻¹ to 400 μg L⁻¹, with a limit of detection (LOD) as low as 4.7 μg L⁻¹ and exhibits excellent selectivity against other competitive pesticides. Moreover, the current chemosensor can generate sensing signals in a short time, which enables it to detect dimethoate pesticide quickly. Further research has revealed the accuracy and reliability of the proposed chemosensor in actual sample detection, which indicates that such a chemosensor will play a potentially vital role in the monitoring of agricultural and food products.