Delay time is crucial in the field application of amended soils. Compaction tests, unconfined compressive strength tests, and direct shear tests were conducted on fly ash-amended loess under varying delay times to analyze the impact of delay time on the mechanical properties of subgrade fill materials. Preliminary microscopic mechanisms were explored through scanning electron microscopy. The results indicate that as delay time increases, the maximum dry density of the fly ash-amended loess initially increases and then decreases, while the optimal moisture content shows a trend of initial increase, followed by a plateau, and then another increase. Both unconfined compressive strength and shear strength are found to gradually decrease with longer delay times, displaying a strong linear correlation with delay time. The cohesion of the amended soil is observed to decrease with increasing delay times, while the internal angle of friction first increases and then decreases, with the reduction in cohesion being more significant than that in the internal angle of friction. Microstructural analysis reveals that with increasing delay times, the overall structure of the amended soil becomes looser, transitioning from larger aggregated particles to more dispersed ones.