Ultraviolet-B (UV-B) light, as an intrinsic part of sunlight, has more significant effects on plant growth and photomorphogenesis than other organisms due to plant's sessile growth pattern. In our studies, we have observed that alfalfa (Medicago sativa L.) seedlings are very sensitive to UV-B performance. Seedlings have grown better at lower levels of UV-B light (UV-B irradiation dosage <17.35 μW cm-2 day-1), and have higher UV-resistance. However, the higher levels of UV-B light (UV-B irradiation dosage >17.35 μW cm-2 day-1) has caused severe stress injuries to alfalfa seedlings, and seriously inhibited its growth and development. Chlorophyll biosynthesis and chlorophyll fluorescence have been suppressed under all different dosage of UV-B light conditions. Plant antioxidant enzymes were induced by lower levels of UV-B, but greatly inhibited under higher levels of UV-B light. The contents of flavonoid compounds significantly increased under UV-B light compared with controls, and that was more significant under lower levels of UV-B than higher levels of UV-B. Therefore, we have assumed that the significant induction of plant antioxidant capacity and flavonoid excessive accumulation play a central role in alfalfa UV-B tolerance to lower levels of UV-B irradiation.