Damage behavior of composite stiffened plates of structure I and structure II at different positions and under different impact energy subjected to low-velocity impact testing was studied in this paper. Visual observation and ultrasonic C-scanning were also employed to inspect the damage size. The results showed that damage behavior of composite stiffened plates was not only related to damage behavior of laminates, but also related to damage between stringer and laminate and damage of stringer itself. It was found that the mechanism of absorbing energy varies with the impact position, as well as the damage behavior. When the impact position was near stringer, partial energy was absorbed by stringer to make stringer and laminate disengage and damage area of laminates was smaller. Damage behavior of composite stiffened plates varies with the structure. Damage area of samples of structure II was smaller than that of samples of structure I. It was estimated preliminarily that design of structure II was better than that of structure I.