A method to sample in situ, separate, enrich and quantitatively measure aldehyde substance content in waters by diffusing of detected substance. Its steps are: (1) Puts some high-molecular compound being capable of occurring chemical reaction with aldehyde substance into device and uses a semi-permeable membrane being able to permeate aldehyde substance to cover it, forms enrich device for sampling and diffusing. (2) Sets device in waters containing aldehyde pollution or substance for 10min-240h. (3) Takes out device to measure aldehyde content in solution by visual spectrometry, high-efficient liquid chromatography or gas chromatography. It has merits of simple, economical, having selectivity, providing concentration and measurement in situ for aldehyde substance in waters, quantitative measurement and dynamics of combined detected substance relates to characteristics of semi-permeable membrane.