Fourier transform infrared spectroscopy (FT-IR) was applied to real-time monitor the reactions of functional groups in polyethyleneimine/glycidyl methacrylate (PEI/GMA) system , to analyze the mechanism and the reactions kinetics. The variation of FT-IR spectra indicates that the primary amine group could react with both epoxy group and alkene group with different reaction rates. Using the transmittance FT-IR spectra of carbonyl group in GMA as a reference, the change of relative amounts and the amounts fluctuations of epoxy and alkene groups were semi-quantitatively determined with the reaction time. The results show that the epoxy group is more active than alkene group in the reaction system. Moreover, between 5 ℃ and 40 ℃ temperature range, heating could accelerate both reactions in favor of the reaction between epoxy and amine group. The results prove that FT-IR is a simple and effective method applied in the studies of reaction mechanisms and is helpful to confirm a suitable reaction condition.