Abstract: The hygrothermal aging tests at 60℃，80℃and RH 90% on F-12 fiber were carried. The moisture absorption behavior law for long hygrothermal aging time was investigated. The effect of absorbed water on the chemical structure and the possible modes of water existing in F-12 fiber under the hygrothermal environment condition were analyzed by ATR-FTIR spectra. The possible moisture absorption mechanism was also discussed. The surface and fracture appearances were observed by SEM，and the deterioration of tensile strength was also studied. The results show that the moisture absorption behavior of F-12 fiber for long hygrothermal aging time belongs to a two-stage model，which accords with Fick‘s second law at the initial stage but deviates at the second stage，and does not reach balance after 2500 h. The molecular structure of F-12 fiber is not destroyed prominently by water. The surfaces of F-12 fiber become rough，on which some grooves appear after 2500 h. Multiple characters of brittleness and fibril split ting are observed around the fracture surface in the anaphase. The tensile strength decreases fluctuating but not continually.