Nano cellulose fibers (CNFs) are recognized as being susceptible to hydrolysis and mineralization in the alkaline environment of cement-based materials, which restricts their broad application in such systems. Nano silica (NS) was combined with CNFs in cement mortar. The synergistic interaction between the two components was examined through compressive strength tests, flexural strength measurements, and fracture toughness evaluations. The addition of NS is shown to significantly enhance the mechanical properties of cement mortar. When 1.5% NS is incorporated, the 90-day compressive strength of the mortar is observed to increase by 22.4% compared to the single additive group. Its fracture toughness is found to improve by 89.1% relative to the single additive group. The microstructure is characterized using low-field nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). Calcium hydroxide in hydration products is consumed through a secondary hydration reaction with nano silica. This process can inhibit the mineralization of nano cellulose. Simultaneously, abundant Ca₅Si₆O₁₆(OH）·4H₂O(C-S-H) gel is generated, resulting in improved matrix density. The bonding capacity between nano fibers and the cement matrix is strengthened. The initiation and propagation of internal microcracks are effectively suppressed. The compactness of the composite material is enhanced through these mechanisms.