The decomposition of submerged macrophytes after senescence is a key process driving the release of internal phosphorus and secondary pollution in lakes. This study selected Vallisneria natans (V), Hydrilla verticillata (H), and Potamogeton wrightii (PW) as research subjects. Using a microcosm incubation method combined with the litterbag technique, we simulated their decomposition processes under both single-species and pairwise mixed-species scenarios. The study systematically investigated the effects of submerged macrophyte decomposition on the transformation of phosphorus (P) forms and its bioavailability in sediments. The results indicated that P in the litter exhibited a net release pattern over the 180-day decomposition period, with the release capacity ranking as H > PW > V. Furthermore, mixed-species decomposition demonstrated a stronger cumulative effect on total phosphorus (TP) than single-species decomposition. The decomposition process significantly reduced sediment pH and oxidation-reduction potential (ORP), promoting the accumulation of various phosphorus fractions, particularly increasing the contents of inorganic phosphorus (IP), iron/aluminum-bound phosphorus (Fe/Al-P), and calcium-bound phosphorus (Ca-P). Concurrently, decomposition significantly elevated the concentrations of bioavailable phosphorus in the sediment, including water-soluble phosphorus (WSP), algal-available phosphorus (AAP), and NaHCO?-extractable phosphorus (Olsen-P), as well as the concentration of labile phosphorus (DGT-labile P) in the sediment. These findings indicate that decomposition exacerbated the mobility of sediment phosphorus and the risk of internal phosphorus loading. These findings suggest that submerged macrophytes decomposition enhances the lability of sediment P and increases the risk of internal P release. The impact of a sediment amendment material was also evaluated. The addition of maifanite promoted P release from V. natans litter and demonstrated potential for regulating P form distribution: it may facilitate the transformation of organic phosphorus (OP) into the more stable Ca-P form while reducing the content of highly labile WSP. This study provides a scientific basis for the rational configuration and biomass management of submerged macrophytes in shallow lakes, as well as for the prevention and control of internal phosphorus pollution risks.