By means of DSC, FITR and static mechanic tests, determined the glass transition temperature Tg and mechanical properties of epoxy resin castings before and after aging. The results showed that the mechanical properties and Tg decreased after 240 h hydrothermal aging and salt-spray test, while after 240 h thermal circulation aging, the mechanical properties and Tg increased. The aging mechanism analysis showed that after hydrothermal aging and salt-spray aging, Tg and mechanical properties decreased owing to the plasticization caused by water molecules immersion, while after thermal circulation aging, the Tg and mechanical properties increased because of cross-linking density caused by post-curing.