The effect of La on flavonoids, chlorophyll contents, and phenylalanine ammonia-lyase (PAL) activity in soybean seedlings under supplementary ultraviolet-B radiation (UV-B, 280 ～ 320 nm) was studied. The results show that PAL activity, contents of flavonoids and chlorophyll in the plants pretreated with La (20 mg· L-1) are higher than those in CK. UV-B radiation could result in an increase in flavonoid content and PAL activity, associated with a decrease in chlorophyll content. However, the increase in the range of PAL activity and flavonoid content in UV-B treatment are lesser than those in the La treatment. The changes of flavonoid contents and PAL activity in La + UV-B treatment are similar to those in UV-B treatment, and the increase in their range is higher than those in UV-B treatment. This shows that La can enhance the resistance of soybean seedling to UV-B radiation and alleviate the damage of UV-B radiation by increasing flavonoid content, chlorophyll content, and PAL activity.