The aim of this research is to investigate the effect of LiNO₃ on the alkali⁻silica reaction (ASR) expansion of reactive sandstone and the mechanism through which this occurs. This paper presents the results from tests carried out on rock prisms and concrete microbars prepared by sandstone and LiNO₃. The findings show that LiNO₃ does not decrease the expansion of these samples unless the molar ratio of [Li]/[Na + K] exceeds 1.66, and the expansion is greatly increased when its concentration is below this critical concentration. The expansion stress test proves that Li₂SiO₃ is obviously expansive. X-ray diffraction (XRD) and scanning electron microscope (SEM) results indicate that LiNO₃ reacts with the microcrystalline quartz inside sandstone, inhibiting the formation of ASR gel, and the formation of Li₂SiO₃ causes larger expansion. A high concentration of LiNO₃ might inhibit the ASR reaction in the early stages, and the formation of Li₂SiO₃ causes expansion and cracks in concrete after a long period of time.