Abstract: The impact dent relaxation characteristic has a crucial effect on probability of visual detection of low-velocity impact damage of aircraft composite structure. Laminate specimens were fabricated using T800 carbon fiber/M21 epoxy resin unidirectional prepreg tow auto-fiber-placing and autoclave curing process. The impact dent of about 1 mm was introduced on specimens, which were subsequently divided into 4 groups. The dent relaxation magnitudes in different periods were measured in room temperature ambient condition, elevated temperature wet condition respectively, cyclic loading condition and cyclic loading in elevated temperature wet condition, to study the effect of hygrothermal environment and cyclic loading on the impact dent relaxation. The results indicate:Hygrothermal environment obviously elevates the final relaxation magnitude of impact dent, yet the effect process is slow, therefore initial relaxation speed is nearly unaffected. Cyclic loading also elevates the final relaxation magnitude of impact dent, the effect of which is relatively insignificant. But it obviously elevates the initial relaxation speed. The effect on impact dent final relaxation magnitude of combination of hygrothermal environment and cyclic loading is larger than either of both, and the initial relaxation speed is almost the same as that in cyclic loading condition.