The chemical structure change and curing behavior of the same thermosetting phenolic resin sample during continuous heating process were studied by in-situ infrared spectroscopy. The results show that there are mainly two reactions during the curing process of the resin,substitution reactions of hydroxymethyl and active hydrogen on benzene ring, condensation reactions between hydroxyl methyl groups, and hydroxyl methyl preferentially reacts with ortho hydrogen on benzene ring. From the continuous changes of infrared spectroscopy curves, condensation reactions between hydroxyl methyl groups build ether bonds, while the occurrence of carbonyl is related to the disappearance of ether bonds.Through continuous real-time monitoring of phenolic group changes during curing process by in-situ infrared spectroscopy, a scientific basis can be provided for the curing procedures of thermosetting phenolic resin.