Analyzing the changes in phosphorus(P) forms of sediments is the key to understanding the mechanisms of sediment P release control. This study investigated the changes in water P concentrations and sediment P-fractions before and after the application of Lanthanum modified bentonite in South Lake in Jinan University in order to analyze the effect on water P concentrations. The results showed that the total P concentration of the lake water decreased by 45.1% after 60 days of water P concentration of the lake changed dramatically after the treatment, after 60 days of the addition of Lanthanum modified bentonite. Meanwhile, the contents of labile-P and reductant-soluble-P were reduced and that of more stable apatite-P and residual-P increased after the treatment, but metal-oxide-P and organic-P did not change. These findings indicate that lanthanum modified bentonite can change sediment P forms and reduce the contents of the potentially releasable sediment P forms which likely contribute to the reduced P concentration in the lake water and thus improve water quality.