The packing effect and pozzolanic effect of superfine slag powder were analyzed to illustrate the packing mechanism of superfine slag powder. In the packing effect, Horsfield model was employed to define the appropriate fineness of slag powder, and the Andrensen equation was introduced to compare the particle size distribution to reveal the contribution of the superfines to the close packing. In pozzolanic effect, the degree of hydration was calculated according to the hydration model and the measured particle size distribution of slag. As an examination of the analysis, the hardened slag cement pastes were tested for strength and mercury intrusion. The results show that the packing effect of the selected powders is positive and obvious; the pozzolanic effect is beneficial to the strength of the paste that is closely related to the degree of hydration calculated.