Abstract: The product of meta phenoxybenzaldehyde and their raw stuff are analysed by gas chromatography/mass spectrometry. The mass spectra of series of compounds and their derivations all showed stronger molecular ions. Sixteen kinds of compounds measured were listed in the paper. According to change of base peak, the prosthetic position of chlorie can be affirmed that is on the benzene or on the methyl. And then the side reaction in the production of meta phenoxybenzaldehyde are derived. Three kind of side reactions are given and it is confirmed from analysis of GC/MS experimental results.