Abstract: Prestressed subgrade is a method suitable for enhancing the existing railway subgrades, and the time-dependent deformation characteristics of the subgrade filler have a direct impact on the long-term stability of prestressed structures. To investigate the creep characteristics of typical subgrade filler, triaxial compression creep tests were conducted on silt filler from the Shuo-Huang Railway, and the creep curves, stress-strain isochronous curves, and time-dependent creep rates were analyzed under different compaction coefficients, under confining pressures and, under stress levels. By introducing fractional-order viscoelastic elements with a viscosity-like coefficient considering stress effects, a nonlinear fractional-order creep model was developed to describe the entire creep development of silt filler, followed by parameter identification and application effectiveness evaluation. The results show that when stress level S_L=0.2~0.8, the creep of silt filler undergoes three stages: instantaneous deformation, decaying creep, and steady-state creep. At S_L=1.0, its creep involves four complete stages: instantaneous deformation, decaying creep, steady-state creep, and accelerated creep. The absolute creep strain significantly increases with the rise in stress level. The stress-strain isochronous curves gradually separate over time, exhibiting remarkable nonlinear characteristics. Both confining pressure and compaction coefficient benefit to the long-term strength of silt filler. The creep rate curves for the first four stress levels experience three stages: instantaneous deformation, decaying creep, and steady-state creep. When the applied stress exceeds its critical failure stress σ_m, the creep rate curve rises rapidly, and ultimately exhibits a U-shape distribution. A comparative analysis on the fractional-order creep model with the Burgers, with the modified Burgers and, with the Merchant models reveal that the creep behavior of silt filler can be effectively characterized by the nonlinear fractional-order model under the step-increase loadings. The model proposed also demonstrates a good fitting on other soil's creep curves, fully validating its accuracy and universality. These findings provide an essential preliminary theoretical basis on establishing a calculation model for the anchorage force loss of prestressed subgrades.