The inherent water absorption characteristics of epoxy resin for plastic sealing cause electrochemical corrosion on the surface of electronic components. The content of polar groups and free volume holes are the main factors affecting the water absorption characteristics of epoxy resin. In view of the problem that the content of polar groups and the distribution of free volume holes are uneven due to insufficient curing in industrial applications, which leads to the change of water absorption behavior of resin, the effect of amine curing agent dosage on the water absorption kinetics and performance of the epoxy resin was studied. In terms of water absorption kinetics, saturated water absorption increased with the increase of amine dosage, but the fitting results showed that the stoichiometric resin had the lowest water diffusion coefficient. The results of in-situ FT-IR showed that water molecules interacted with hydroxyl and carbonyl groups during water absorption. Under saturated water absorption, the glass transition temperature (Tg) of epoxy-rich resin decreased by 16.7%, that of isostoichiometric resin decreased by 16.6%, and that of amine-rich resin decreased by 32.3%. In terms of mechanical properties, due to water absorption, the yield strength of epoxy-rich resin decreased by 3.9%, that of isostoichiometric resin decreased by 5.1%, and that of amine-rich resin decreased by 8.5%. After secondary drying treatment, the Tg, compressive modulus and compressive strength of isostoichiometric resin and amine-rich resin recovered well. The experimental results show that insufficient curing caused by the blending ratio has a significant effect on the water absorption behavior of epoxy resin.