Oils and fats undergo rancidity reaction under the action of light, heat and oxygen in the air, and compounds such as aldehydes and acids are decomposed. Malondialdehyde (MDA) is one of the decomposition products, which can react with TBA (thiobarbituric acid) to generate pink compounds, and has an absorption peak at the wavelength of 532nm. Using this property, the content of MDA can be measured, and the degree of rancidity of oil can be deduced.