Abstract: Taking powders of steel slag (SS) and blast furnace slag (BFS) as main raw materials and P.O 52.5 cement as alkali-activator, the binding materials and its mortar sample were prepared with the secondary mixed grinding method. The properties of mortar sample were characterized and analyzed. The results show that the compressive strength (CS), flexural strength (FS) and softening coefficient (SC) of steel slag cementitious materials at different ages first increase and then decrease with the increase of P.O 52.5 cement content, when the content of aluminosilicate raw material is 4% (mass fraction, the same below) and the mass ratio of SS to BFS is 1:1. When the content of P.O 52.5 cement is 24%, the CS, FS and SC of mortar sample at different ages reach the maximum. The CS and FS at 3 d are 18.2 and 4.8 MPa respectively, which meets the strength index requirement of steel slag portland cement with 42.5 strength grade. The CS and FS at 28 d are 36.8 and 8.1 MPa respectively, which meets the strength index requirements of steel and iron slag cement with 32.5 strength grade.