Abstract: Excess phosphorus input into lakes always leads to eutrophication even to cyanobacterium bloom. Extracellular alkaline phosphatase of bacterioplankton plays an important role in phosphorus cycling, while its relations with particulate organic matter and bioavailable phosphorus remain unclear. In this study, the 3 strains of organic phosphorus mineralizing bacteria (OPB) isolated from the sediment of Lake Taihu together with the natural OPB assemblage in Lake Donghu were used to detect relations of extracellular alkaline phosphatase activity (APA), including its sizes fractionations, with OPB abundances and phosphorus concentrations in different forms in the simulation experiment with cyanobacterium detritus as particulate organic matter. In general, APA showed a significantly positive relationship with the OPB numbers, and a significantly negative one with the soluble reactive phosphorus (SRP) concentration. Along the course of the experiment, the three strains and those in the nature assemblage responded to significantly different SRP concentrations showing their distinct abilities to mineralize organic phosphorus. Among the treatments containing different OPB strains the coarser (>3.0 μm) and dissolved (8 CFU/mL. In the last stage (20—33d) of the experiment, the OPB numbers drastically decreased, accompanied by a marked increase of SRP concentration and dissolved organic phosphorus concentration. Taken together, under the conditions of phosphorus deficiency and the enrichment of particle organic matter, OPB would produce extracellular alkaline phosphatase to decompose organic phosphorus to compensate phosphorus deficiency for their active growth, and change the form and bioavailability of soluble phosphorus effectively.