A simple and effective method based on in situ infrared spectroscopy and two-dimensional (2D) correlation analysis was applied to research the chemical changes and curing reaction mechanism of epoxy resin and amine curing agents. It is generally agreed that the epoxy groups in epoxy resin react with amino groups to form new C–N and hydroxyl groups during the curing reaction process. However, detailed information about the curing reaction mechanism of epoxy resin and amine curing agents has rarely been reported. In this work, the curing reaction mechanism can be deeply understood from the results of 2D correlation analysis. Due to the nucleophilic addition reaction of amino and epoxy groups, the nitrogen atoms of primary amines easily combine with the carbon atoms in epoxy groups, which forms new C–N groups. Then, the C–O bonds in epoxy groups break; finally, as the N–H bonds in primary amines break, the hydrogen atoms combine with the oxygen atoms to form new hydroxyl groups.