The influences of slag ultrafine powder (SUP) on such cement hydration products as ettringite (AFt), Ca(OH)₂ and CSH gel under different hydration time spans were studied by using XRD, TG-DSC and ESEM techniques. The results show that SUP participates in the cement hydration process and remarkably affects the formation rate, product content and morphology of hydration products. Moreover, with increasing SUP dosage and prolonging hydration time, SUP gradually further affects the cement hydration products.