Bloom-Cyanobacteria can release phosphorus (P) into overlying water during their decline period, thus inevitably providing available P for the next round of bloom. In order to quantitatively evaluate the contribution of cyanobacterial bloom decline to P amounts in Caohai, a typical cyanobacteria-dominated sub-lake in northern Dianchi Lake, the P concentrations in algae during the peak and bottom of cyanobacterial bloom were measured and calculated. Remote sensing monitoring analysis and monthly monitoring data showed that the cyanobacterial bloom in Caohai developed from June, reached its peak in July to August and then declined to its bottom from December to February. The concentrations of different phosphorus forms contained in algal cells were different between the peak and bottom of the cyanobacterial bloom. Total phosphorus (TP) concentration in algae (TP-A) were higher in summer than in winter, while the TP content per unit Chl-a in winter was much larger than in summer. The annual released TP was approximately 24.12 tons in 2016, and its potential contribution to TP and Ortho-P in water body of Caohai was around 0.958 and 0.647 mg⋅L^-1, respectively. The P release amount was 303.30 and 20.57 tons in 2011 and 2014, respectively. For Caohai of Dianchi Lake, the P released from bloom-cyanobacteria could provide adequate P for the next year’s bloom recovery.