In order to explore the proportion of the steel slag powder and mineral admixture complex composition, in this experiment, the volume stability of the steel slag powder and the mixed cementitious system was tested by the test cake method and the Lei folder method. The suitable grinding time of steel slag and the proportion of steel slag powder were selected by mechanical property characterization, and the particle size distribution of cemented particles was compared with preferred group of strength characterization of mixed cementitious system. The results showed that the steel slag proper grinding time should be 30 min and the volume stability can meet specification requirements when the slag powder and mineral admixture are mixed. When steel slag powder and slag powder mix at the ratio of 1∶1, accounting for 40% of cementitious material,steel slag powder and fly ash mix at the ratio of 2∶[KG-*3]1, accounting for 30% of the cementitious material and steel slag powder, steel slag powder, fly ash and slag powder mix at the ratio of 2∶1∶1, accounting for 40% of cementitious material, the mechanical properties and particle size distribution of the above three kinds of mixed cementitious system are excellent.