C-S-H is the main hydration product of common Portland cement, which plays a very important role in the properties of cement-based materials. However, the hydration products of cement are complex, and it is difficult to separate pure C-S-H from the hydration products and explore its influence on cement-based materials. Therefore, nano C-S-H (NC) particles were prepared by the double decomposition method in this paper and mixed into the slag powder-cement system. The influence of NC on the hydration of slag powder-cement system was investigated by means of non-contact electrical resistivity measurement, XRD, DSC-TG, SEM, and MIP. Results show that in the range of 1% to 4% (by mass) dosage, the addition of NC shortens the setting time of matrix, and provides more nucleation sites for cement hydration, accelerates the formation and precipitation of hydration products. It also promotes the bond between the hydration products, thus reducing the porosity and increasing the early strength and resistivity of cement matrix composites.