Using total phosphorus (TP) concentrations to estimate eutrophication risk is problematic for management purposes, as only some forms of P are biologically available for phytoplankton growth. This study estimated the bioavailability of P fractions of water and the floodplain sediment in the lower reaches of the Hanjiang River in China using multiple regressions. And the effects of concentrations of P fractions on algal growth were also studied. The results showed that the total soluble P (TSP) and the particulate P (PP) can well predict the growth of Chlorella pyrenoidosa in water samples. And the exchangeable P (exch-P) and the P bound to Al, Fe, and Mn oxides and hydroxides (Fe/Al–P) were adapted to predicting the growth of Chlorella pyrenoidosa in the floodplain sediment samples.