Calcium nitrate (CN) is used widely as an effectively inorganic setting accelerator and antifreeze admixture in concrete structures. In this paper, the multiscale investigation of CN on the hydration of C₃S was studied by scanning transmission X-ray microscopy (STXM) combined with near-edge X-ray absorption fine structure (NEXAFS), ²⁹Si MAS NMR, calorimetry, scanning electron microscope, and N₂ absorption. It was concluded that the calcium silicate hydrate (C-S-H) surrounds the unhydrated C₃S at 1-day hydrated C₃S in the presence of calcium nitrate, while portlandite is partly in transformation and is formed partly. Based on Ca L_3,2-edge NEXAFS spectra for 1-day hydrated C₃S particle, calcium nitrate does not change the structure of the asymmetrically 7-fold coordination of calcium in the C-S-H. Calcium nitrate can accelerate the hydration of C₃S to some extent and polymerization of the silicate chains within C-S-H considerably at early age, resulting in the increasing specific surface area.