Aim & Goff model was used to predict the packing density of cementitious material including cement, ultra-pulverized fly ash and silica fume. The mix proportions of reactive powder concrete (RPC) with different packing density and calcium-silicon ratio of cementitious material were designed, and a strength test was carried out. The study results reveal that the flexural strength and compressive strength of RPC are related to the packing density and calcium-silicon ratio of cementitious material. For the mix proportion of RPC with the calcium-silicon ratio of 1.179, calcium hydroxide reacts with silicon dioxide fully, and the superfluous ultra-pulverized fly ash and silica fume fill the voids of RPC. The packing density of its cementitious material is the largest, and its strength approaches summit.