Abstract: Methods of qualitative and quantitative analysis of the by-product in the course of cyclohexone production were established by high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Firstly, the variety involved in the sample was tested quantitative by MS/MS, of which the main components were adipic acid, pentanedioic acid, butanedioic acid and the products of their dehydration. The sample was seperated on Hypersil GOLD C₁₈ with 40% methanol and 60% water (pH was tuned up to 3.5 by formic acid) as the mobile phase. Detection was performed on a tandem mass spectrometry in selective reaction monitoring (SRM) mode. Then peak areas were quantitated. The results indicate that the linear range of adipic acid, pentanedioic acid and butanedioic acid are 32-600 μg•kg^-1, 8-150 μg•kg^-1, 8-150 μg•kg^-1 respectively with correlation coefficients of all stadard curves more than 0.995 0. The content of adipic acid, pentanedioic acid and butanedioic acid in the sample are 80.140%, 7.316% and 6.170% respectively with relative standard deviation of all samples less than 8％. This method is sensitive, accurate and will be apply to the determination of adipic acid, pentanedioic acid and butanedioic acid.