Based on the fact that hydrogen cyanide reacts with ninhydrin hydrate in alkaline condition,and resulting to the solution turning purple red,a rapid and sensitive method was developed for on site detection of hydrogen cyanide in water sources.The factors such as type of chromogenic agent,concentration of ninhydrin hydrate,pH value,reaction temperature and time were optimized.The RGB colorimetric method originated from the normalized RGB system were used in the detection,overcoming the problem that three stimulus values changed with the variation of light intensity based on the RGB model.Under the optimum conditions,there were good linear relationships between chromaticity values and hydrogen cyanide concentrations in the range of 0.04-3.96 mg/L.The theoretical detection limit for R value was 0.038 mg/L and the actual detection limit was 0.04 mg/L.The method was successfully applied in the determination of hydrogen cyanide in artificial water samples with recoveries of 98.3%-111%.With high selectivity,sensitivity and reproducibility,the method also had an application potential in the analysis of other toxic and harmful substances in environment.