Abstract: HT9-ODS steel exhibits an anomalous creep weakening phenomenon compared to the matrix HT9 steel under high-temperature (650 ℃) and high-stress conditions. To elucidate the underlying deformation mechanisms and their correlation with performance degradation, this study aims to quantify the contributions of dislocation slip, grain boundary sliding (GBS), and Coble creep to the macroscopic plastic deformation and texture evolution. To achieve this, an improved visco-plastic self-consistent (VPSC) model was constructed and calibrated using experimental creep data obtained at 650 ℃ under applied stresses of 100, 140, and 180 MPa. The model integrated dislocation slip, GBS, and Coble creep as distinct rate-sensitive mechanisms. Specifically, the dislocation slip was described by an extended Voce hardening law, while the strengthening effect of oxide nanoparticles was incorporated via the dispersed barrier hardening (DBH) model. Simulations were conducted on both HT9-ODS and HT9 steels to decouple the influence of oxide dispersion and grain boundary characteristics. The activation of specific slip systems (including 110〈111〉 and 112〈111〉) and the lattice rotation paths were analyzed systematically. The simulation results exhibit good agreement with the experimental data, particularly in capturing the steady-state creep rates and the texture evolution trends. The model effectively reproduces the deformation behavior during the primary and secondary creep stages. The study reveals that, macroscopically, the steady-state creep rate of HT9-ODS steel is significantly higher than that of the HT9 matrix under the investigated stress range (100-180 MPa). Micro-mechanism analysis indicates that while dislocation slip remains the dominant mechanism in both materials, the role of grain boundary mechanisms differs fundamentally. A critical finding is that the absolute strain rate contributed by GBS in HT9-ODS steel is markedly higher than that in the HT9 matrix. Although the current VPSC model does not explicitly simulate damage evolution, this excessive GBS activity is inferred to aggravate the strain incompatibility at the oxide/matrix interface, providing kinetic conditions for interfacial void nucleation. Furthermore, although theoretical analysis suggests that HT9-ODS steel possesses a “harder” texture orientation, the results show a diffuse texture feature. Detailed analysis attributes this to the asymmetric activation of slip systems; specifically, the dominance of the 112〈111〉 slip system alters the lattice rotation path, preventing the formation of sharp textures. In conclusion, the comprehensive analysis suggests that the creep weakening of HT9-ODS steel is not governed by geometric softening. Instead, the “physical softening” effect, induced by the evolution of micro-physical mechanisms (particularly the excessive GBS), outweighs the geometric hardening contribution from crystal orientation. This micro-mechanism competition is identified as the dominant factor leading to the degradation of high-temperature creep performance. This work provides a systematic framework for the constitutive modeling and mechanism analysis of ODS steels.