We treated Columbia-0 wild-type Arabidopsis thaliana seedlings with enhanced UV-B radiation (16.67 μW·cm^-2 radiant power) at 0.5, 1, 1.5, 2, 2.5, and 3 h. We observed leaf morphological characters and detected root length, leaf chlorophyll content, soluble protein content, malondialdehyde (MDA) concentration, superoxide dismutase (SOD) activity, catalase (CAT) activity and chlorophyll fluorescence characteristics. With a short period of UV-B radiation, roots were elongated and chlorophyll content and soluble protein content were increased in leaf, but prolonged UV-B radiation inhibited the elongated root length; caused decreased leaf chlorophyll content, soluble protein content and chlorophyll fluorescence characteristics such as F_v/F_m and qP; promoted MDA concentration; and increased SOD and CAT activity and qN over time. With 16.67 μW·cm^-2 radiation, the optimal time of UV-B radiation was 1.5 h. If UV-B radiation acts as a kind of environmental stress, the extent of damage to the plant was restricted, and the plant would have to adapt to UV-B radiation to let the damage reach a minimum when the stress level reached the limitation.