Abstract: 2,3,5,6-Tetrachloro-4-methylsulfonylpyridine are identified by gas chromatography-mass spectrometry (GC/MS); The fragment routes of characteristic are discussed on the basis of EI mass spectra. The characteristic of the compound is of C-S bonds fragmentation of sulphonyl groups, C-Cl bonds and O-C bonds fragmentation accompanied by hydrogen migration of methyl sulphonyl groups. The full scan MS2 spectra of fragment m/z 231 is obtained by ion trap mass spectrometry in a multi-stage tandem MS full scan mode. The characteristic fragments of m/z 231 are used to prove the structure of m/z 231.