The　unclear　understanding　of　the　water　diffusion　behavior　posts　a　big　challenge　to　the　manipulation　of　water　absorption　properties　in　epoxy　resins.　Herein,　we　investigated　the　water　diffusion　behavior　and　its　relationship　with　molecule　structures　inside　an　epoxy　resin　mainly　by　the　nonequilibrium　molecular　dynamics　and　experiments.　It　is　found　that　at　the　initial　rapid　water　absorption　stage,　bound　water　and　free　water　both　contribute,　while　at　the　later　slow　water　absorption　stage,　free　water　plays　a　dominant　role.　The　observed　evolution　of　free　water　and　bound　water　cannot　be　explained　by　the　traditional　Langmuir　model.　In　addition,　molecule　polarity,　free　volume,　and　segment　mobility　can　all　influence　the　water　diffusion　process.　Hence,　the　epoxy　resin　with　low　polarity　and　high　molecular　segment　mobility　is　endowed　with　higher　diffusion　coefficients.　The　saturated　water　absorption　content　is　almost　dependent　on　the　polarity.　The　understanding　of　how　water　diffuses　and　what　decides　the　diffusion　process　is　critical　to　the　rational　design　of　molecule　structures　for　improving　the　water　resistance　in　epoxy　resin.