[Objective] This study aims to explore the effect of phosphorus on the fluid variability characteristics of sediment-laden flows. [Methods] A self-made double vertical tube rheometer was used to study the influence of phosphorus on fluid types, rheological parameters, and the critical threshold of fluid variation in sediment-laden flows with different concentrations. [Results] (1) Phosphorus concentration, sediment concentration, and physical and chemical properties of sediment were the main factors influencing the fluid variability in sediment-laden flows. At 25 ℃, the viscosity coefficient and Bingham yield stress increased by 0.48~1.47 mPa·s and 3.49~6.84 N/m², respectively, with an increase of 0.1 g/cm³ in calcium superphosphate concentration. (2) With the increase of phosphorus and sediment content, phosphorus-laden sediment-laden flows transitioned from Newtonian fluid to Bingham fluid. A computational model was developed and validated to calculate rheological parameters such as viscosity coefficient and Bingham yield stress. (3) The critical concentration threshold for the fluid variability of phosphorus-sediment-laden flows was given. When phosphorus concentration increased from 0 to 0.45 g/cm³, the critical concentration threshold for fluid variability in sediment-laden flows decreased by 49%, indicating that the presence of phosphorus accelerated the fluid variability of sediment-laden flows. Therefore, the increase of phosphorus promoted the process of erosion water flow from Newtonian fluid to Bingham fluid, which affected the energy dissipation process in the erosion flow. [Conclusion] The research findings provide a new scientific basis for in-depth understanding of the transport mechanism of erosive polluted water flows.