To study the promotion process of cement hydration by polycarboxylate superplasticizers modified calcium silicate hydrate (C-S-H/PCE) and understand the mechanism of C-S-H/PCE seeds promotion of cement hydration. Acrylic acid-based polycarboxylate superplasticizer (PCE-1) and silane-based polycarboxylate superplasticizer (PCE-2) were used to surface modify the hydrothermal synthesized calcium silicate hydrate (C-S-H/Unmodified) seeds, resulting in the polycarboxylate superplasticizer modified C-S-H seeds (C-S-H/PCE-1 and C-S-H/PCE-2). Subsequently, these three C-S-H seeds were added to the cement paste. The compressive strength, phase composition, and heat release curve of cement paste and the surface tension of C-S-H seeds were tested. The results showed that the surface modification of PCE-1 and PCE-2 resulted in an increase in the surface contact angle of C-S-H seeds from 36.9° to 41.1° and 42.9°, resulting in lattice mismatch of the sedimentary phase (hydration product) on the surface of the substrate material (C-S-H seeds) and inhibiting the epitaxial growth of cement hydration products on the surface of C-S-H seeds. PCE modification reduced the promoting effect of C-S-H seeds on the hydration process of Portland cement. Moreover, the degree of reduction in the hydration promoting effect of C-S-H/PCE-2 on Portland cement is significantly greater than that of C-S-H/PCE-1 crystal nucleus. Tests of the adsorption equilibrium of PCE in the pore solution indicated that the C-S-H/PCE seeds did not directly introduce new crystal nucleation sites but exposed new C-S-H surfaces through the desorption of PCE on the surface of C-S-H/PCE seeds. The desorption degree of PCE on the surface of C-S-H/PCE seeds determined the promoting effect of C-S-H/PCE seeds on cement hydration.