Abstract: The underlying causes of strength deterioration for 9%Cr martensitic heat-resistant steels during high-temperature thermal aging are still debated. The mechanisms of microstructure-driven strength deterioration in 2.6W-G115 steel during thermal aging at 650 ℃ within 11,000 h were systematically investigated by combining ex situ synchrotron radiation X-ray diffraction and chemical phase analysis. The degradation of solid solution strengthening is dominated by C element and W element. The maximum peak of precipitation strengthening occurs at 500 h aging, and M₂₃C₆ carbides exhibit more pronounced pinning effect on martensitic subgrain boundaries. The strength deterioration is most concentrated during aging between 100 and 5000 h, though the microstructural degradation mechanisms vary considerably at different aging stages. During 100-500 h aging, the nucleation and growth of precipitates and the recovery of unstable dislocations predominate. The Hall-Petch strengthening undergoes significant degradation due to the broadening of martensitic laths during 500-1000 h aging. In the later stages of thermal aging (1-5000 h), the degradation of microstructure is governed by solid solution strengthening, precipitation strengthening, and Hall-Petch strengthening. A yield strength prediction model was successfully established based on microstructure degradation by coupling different strengthening mechanisms, which matches well with the experimental data.