Low temperature environment affects the development of early performance of tunnel shotcrete. This study investigated the effects of silica fume on the fluidity, compressive strength, and impermeability of alkali-free acceleratormodified cement mortars at low temperatures. XRD, TG-DTG and MIP were used to analyze the changes in hydration products and pore structure. The results show that using silica fume significantly improves the 1 d compressive strength of mortar at low temperatures with an increase of over 300. 0%. At 28 d, incorporating 5% (mass fraction) silica fume also increased the compressive strength by 4. 7%. Moreover, the impermeability of mortar increases with the increase of the silica fume content. Especially, the impermeability is remarkably improved when the content of silica fume exceeded 10%.Silica fume modification can exert the pozzolanic effect and micro-filling effect at low temperatures, which transformes moreharmful pores towards less-harmful ones, refining the pore structure and making the paste structure denser, thereby improving the compressive strength and impermeability of mortars.