In the paper, the influence of the particle size distribution in binary system on the packing density and the rate of hydration is analysed. On the basis of the analysis, the relationship between the porosity and the particle size distribution is calculated and discussed. The result indicates that: (1) The influence of the particle size distribution on the porosity is dependent on both the packing density and the rate of hydration. When the depth of hydration is less, the packing density is the main factor, thus the wide distribution can obtain the low porosity; when the depth of hydration is greater, the influence of both must be considered comprehensively. There is the best distribution, and the best value of d 1 /d 2 is about 0.3 for binary system. (2) The relative ratio between two components depends mainly on the packing density. The mixture ratio when the porosity is the lowest equals to one when the packing density is the highest.