The immobilization of sedimentary phosphorus (P) by capping sediments with amended calcium peroxide material (ACPM) and CaO2 was investigated, the sediments and overlying water from a black-odor river. Under the capping with ACPM and CaO2, the pH and dissolved oxygen (DO) in the overlying water were higher than in the control. This contributed to the more loss of external P under the ACPM (the phosphate sorption was fitted well by the Langmuir equation, the maximum P sorption capacity up to 29.28 mg P g(-1)). Additionally, ACPM improves the decrease of the reduction materials (e.g., NH4+ -N, Fe2+) in the pore water compared to that associated with CaO2, resulting in a less obvious dissolved inorganic P (DIP) in the pore water, but the accumulation of DIP in the pore water with CaO2 suggested an explosive release once the micro-environment conditions encounters adverse conditions. The sharp reduction of Fe/Al-P and Res-P contributes to the increase of DIP in the pore water, with a release of up to 390.60 mg P kg(-1) DW (dry weight), whereas the value is only 62.55 mg P kg(-1) DW when capped with ACPM. Correspondingly, the percentage of the bioavailable P to Tot-P decreases rapidly from 17.27% (control, 0-2 cm) to 7.21% (ACPM, 0-2 cm) and 10.57% (CaO2, 0-2 cm), indicating the obvious P immobilization. The implication is that the application of CaO2 is not suitable for direct use, while the combination with other materials (e.g., WPS) is a better choice.