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A type of flexible epoxy resin was synthesized by a two-step reaction, in which isophorone diisocyanate was first reacted with the hydroxyl group of epoxy resin to obtain a modified epoxy resin with the --NCO side groups and then polypropylene glycol (PPG) was grafted onto the side chains of the modified epoxy resin through the reaction between the hydroxyl groups of PPG and the --NCO groups on the modified epoxy resin to obtain polyether-grafted epoxy resin (PPG-g-EP) with different molecular weights (PPG400, PPG1000 and PPG2000). The structure of the resultant PPG-g-EP was characterized by titration and FTIR. The thermosetting PPG-g-EP was further prepared by using polyetheramine as a curing agent, and its mechanical properties were measured by a universal testing machine. The results indicated that the thermosetting PPG-g-EP with a longer PPG chain achieved higher elongation at break, and the PPG2000-modified epoxy resin gained the maximum elongation at break of 98 %. [ABSTRACT FROM AUTHOR]