Abstract: The great challenge of cemented tailings backfill (CTB) is difficult simultaneously maintaining its excellent mechanical properties and low cost. Fly ash (FA) can potentially address this problem and further replace cement in favor of low carbon development. However, its mechanism on CTB with low cement dosage and low Ca system remains unclear. Consequently, this study conducted uniaxial compression, X-ray diffraction (XRD), and scanning electron microscopy (SEM)–energy dispersive spectrometer (EDS) tests to investigate the effect of FA dosage on the mechanical property and microstructure of CTB. A molecular model of FA-CSH was constructed to reproduce the molecular structure evolution of CTB with FA based on the test results. The influences of FA dosage and calcium/silica molar ratio (Ca/Si ratio) on the matrix strength and failure model were analyzed to reveal the mechanism of FA on calcium silicate hydrated (C–S–H). The results show that the strength of CTB increases initially and then decreases with FA dosage, and the FA supplement leads to a decrease in Ca(OH)₂ diffraction intensity and Ca/Si ratio around the FA particles. XRD and SEM–EDS findings show that the Ca/Si ratio of C–S–H decreases with the progression of hydration. The FA-CSH model indicates that FA can reinforce the silica chain of C–S–H to increase the matrix strength. However, this enhancement is weakened by supplementing excessive FA dosage. In addition, the hydrogen bonds among water molecules deteriorate, reducing the matrix strength. A low Ca/Si ratio results in an increase in water molecules and a decrease in the ionic bonds combined with Ca²⁺. The hydrogen bonds among water molecules cannot withstand high stresses, resulting in a reduction in strength. The water absorption of the FA-CSH model is negatively correlated with the FA dosage and Ca/Si ratio. The use of optimal FA dosage and Ca/Si ratio leads to suitable water absorption, which further affects the failure mode of FA-CSH.