Soil phosphorus (P) is an essential nutrient that controls wetland productivity and ecological functions. However, the effects of soil P forms on P availability during wetland degradation are relatively unknown. Soil samples from differently degraded marshes, including relatively pristine marsh (RPM), lightly degraded marsh (LDM), moderately degraded marsh (MDM), and heavily degraded marsh (HDM), were collected to investigate the changes in soil P forms and its regulation on P availability in the Zoige Plateau, China. We observed that compared with RPM, the main changes in total P concentration were a significant increase of 31.6%–44.2% in the 0–30 cm soil layers of LDM and MDM, and the available P concentration increased in LDM and MDM but decreased in HDM with a lower P activation coefficient. Marsh degradation increased the concentration and proportion of dicalcium phosphates, P occluded in iron hydroxides, and organic P but decreased those of iron oxide surfaces adsorbed P and apatite P. Soil available P was mainly related to organic P and P non-occluded in iron oxide minerals that might also be non-negligible direct source of available P. The transformation from apatite P to organic P was an important regulation mechanism of P availability in soils during marsh degradation. This study revealed the risk of P limitation in heavily degraded marsh soils and established the mechanism by which marsh degradation significantly influences soil P availability. Therefore, some measure of on improving P availability should be implement for the ecological restoration of heavily degraded marsh in the future, such as grazing exclusion and the application of organic fertiliser.