This paper was to synthesize C-S-H with different Ca/Si ratios by a hydrothermal synthesis method, and decalcify it using 6 mol/L NH4Cl solutions. The chemical composition, chain structure and microstructure were analyzed. The results show that C-S-H is a gel structure formed by irregular aggregates. The content of dimer and chain end groups in C-S-H increases, while the average degree of polymerization and average chain length decreases with the increase of Ca/Si ratio. The change of C-S-H structure includes decreased interlayer distance, cross-linked silicate chains, and amorphous silica gels formed after decalcification, resulting in the loss of C-S-H cohesion.