Dynamic state of antioxidant capacity of flavonoids was investigated for a further demonstration of alleviating the damage of the UV-B radiation in the La-treated soybean seedlings under UV-B stress. Using hydroponics culture, the effects of lanthanum on the contents of flavonoids and its ability of antioxidant under elevated ultraviolet-B radiation (280-320 nm) was studied in this paper. The results showed flavonoids contents in Soybean seedlings during the stress and convalescent period increase firstly and then reduce. Membrane permeability and MDA contents increase firstly (1st-5th day) and then fall (6th to 11th day). A similar change of flavonoids contents and clearance of flavonoids scavenging O2*- and *OH in soybean seedlings occur; the flavonoids contents La(III) + UV-B > UV-B > La(III) > CK, La(III) + T1 > La(III) + T2. Plasma membrane permeability and MAD contents UV-B> La(III) + UV-B > CK > La(III), La(III) + T2 > La(III) + T1. The ability of Scavenging activities of free radical (O2*-, *OH) La(III) + UV-B > UV-B > La(III) > CK. It suggested that the regulative effect of La(III) on flavonoids, improved the metabolism of ROS, diminished the concentration of MDA and maintained normal Plasma membrane permeability, and that its protective effect against low UV-B radiation was superior to that of high UV-B radiation. To conclude, the defensive effect of La(III) on soybean seedlings under UV-B stress was carried out on the layer of defense system.