Nano-sized C-S-H crystal seed nucleating agent can effectively promote the hydration process of cement and improve the early strength of cementitious composites via providing some nucleation sites for cement hydration products. The calcium-silica molar ratio can affect the early strength effect of nano-sized C-S-H crystalline seeds on cement, but the specific influence law is still controversial. In this paper, the effect of calcium-silica molar ratio on the size and morphology of nano-sized C-S-H crystalline species and their early strength effects on cement was investigated by X-ray fluorescence spectroscopy, multiple light scattering analysis, dynamic light scattering analysis, X-ray diffraction, transmission electron microscopy, cement hydration exotherm and strength tests, and the mechanism of calcium-silica molar ratio affecting the early strength of nano-sized C-S-H crystalline species on cement was elucidated. The results show that the nucleation efficiency of C-S-H nano-crystals can be improved as calcium and silicon molarity is increased.The size of C-S-H nano-crystals gradually becomes smaller and the dispersion stability of suspension improves when calcium-silica molar ratio increases from 1.0 to 1.7. Also, the morphology of C-S-H nano-crystals changes from spherical to tin foil, and the stacking between the crystals decreases and the layer thickness becomes thinner when calcium-silica molar ratio increases. The C-S-H nano-crystals can promote the development of cement hydration exothermic rate and early strength, especially within the age of 1 d. The higher the calcium-silica molar ratio is, the greater the acceleration of cement hydration exothermic rate by C-S-H nano-crystals and the higher the early compressive strength enhancement will be.