Ethylene-vinyl acetate copolymer (EVM) vulcanizates with good mechanical properties and high water-swellable performance were prepared by in situ formed sodium acrylate (NaAA). The effects of NaAA content on the mechanical properties, crosslink structure and water-absorptivity of EVM vulcanizates were studied. The tensile strength and tear strength of EVM vulcanizates greatly increased with increasing NaAA content. Crosslink density analysis indicated that the EVM vulcanizates contained both covalent bonds and ionic bonds. Ionic crosslink density was greatly increased by increasing the amount of NaAA. FTIR analysis confirmed that NaAA formed in the process of compounding and underwent polymerization during vulcanization. The EVM vulcanizates reinforced by NaAA had high water absorbing rate. When the NaAA content was 50 phr, the maximum water-absorbing ratio of EVM vulcanziate was 278%. In the case of constant NaAA content, the tensile strength of EVM vulcanizate firstly decreased and then remained almost constant with the increase of swelling ratio, while the elongation at break decreased first and then gradually increased.