Abstract: Formaldehyde, acetaldehyde, acrolein and trichloroacetaldehyde have been a focus in the environment because of their toxicity and harmfulness as potential carcinogens. The four aldehydes were listed in the environmental quality standard for surface water in China and were monitored each month by the environmental monitoring department. According to the standard methods for determination of the four aldehydes, formaldehyde was determined by spectrophotometry and acetaldehyde, acrolein and trichloroacetaldehyde were detected by Gas Chromatography. A new analytical method has been developed for the determination of the four aldehydes by High Performance Liquid Chromatography (HPLC) after the 2,4-dinitrophenylhadrazine (DNPH) derivatization. The four aldehydes were separated by HPLC with a ternary gradient program, which mobile phase changed by linear gradient from pump A (Φwater=100%) 40%, B (Φmethanol=100%)50%, C (Φacetonitrile=100%) 10% to pump A (Φwater=100%) 20%, B (Φmethanol=100%) 60%, C (Φacetonitrile=100%) 20% in 15 min. Effect factors of acidity, temperature and reaction time on derivatization were studied, and the conditions for separation of four aldehydes were optimized. The water sample initially reacted with DNPH for one hour under the condition of pH=3 and 40℃, and then was extracted by dichloromethane. The recoveries of four compounds were in the range of 73.1%-115%, relative standard deviations were in the range of 0.71%-5.30%, and the detection limits were in the range of 3.50-28.6 μg/L. This method is simple, rapid and sensitive for the determination of the four aldehydes in surface water.