•An AuNPs based colorimetric method is developed for organophosphous pesticide analysis.•The detection mechanism is based on dissolution of AuNPs and AChE hydrolysis reaction.•The colorimetric method has high sensitivity and good stability.•The colorimetric method could be applied in aqueous solutions with high salinity. A simple and sensitive colorimetric method is developed for organophosphrous pesticides (OPs) analysis based on the enzymatic hydrolysis reaction of acetylcholinesterase and the dissolution of gold nanoparticles (AuNPs) in Au3+-cetyltrimethylammonium bromide (Au3+-CTAB) solution. In the absence of OPs, the enzymatic hydrolysis product, thiocholine, would reduce Au3+ and protect the AuNPs from dissolution by the Au3+-CTAB. In the presence of OPs, however, the activity of AChE is inhibited, which could not or could only produce a small amount of thiocholine to consume the Au3+. In this case, the large amount of residual Au3+ dissolves the AuNPs and decreases both the concentration and size of the AuNPs, thus leading to an obvious red-to-light pink or red-to-colorless color change. Under optimal conditions, the colorimetric method could indicate the presence of OPs, with the concentration down to 0.7ppb. After loading AuNPs on a cellulose paper, an AuNPs-coated dipstick is developed for the detection of OPs, which is highly sensitive with an observable limit of detection of 35ppb. Practical sample analyses in tap water, apple washing solution, and sea water indicate the colorimetric method has acceptable accuracy and good stability, even in the system containing 5M of NaCl. Regarding these merits, the as-proposed method could be applied for the point-of-care analysis of OPs in complex systems.