In order to investigate synergistic cementing effect between binary solid waste, the mechanical properties of the steel slag-red mud-cement based composite mortar were investigated under different ratio conditions, and the hydration characteristics and microscopic morphology of composite mortar were characterized by hydration heat, XRD, TG-DTG, SEM, etc. The study results show that compared with pure cement group, single addition 30% (the following are mass fractions) steel slag inhibits hydration reaction of slurry, and reduces mechanical properties of mortar. On the basis of adding 30% steel slag alone, adding an appropriate amount of red mud can effectively reduce the negative impact of steel slag on the mechanical properties of mortar. The 28 d flexural strength and 28 d compressive strength of composite mortar are the highest when the steel slag is mixed with 15% and the red mud is mixed with 15%, which are 6.8 and 39.8 MPa, respectively. Compared with the group with single addition 30% steel slag, the 28 d flexural strength and 28 d compressive strength increase by 11.5% and 20.6%, respectively, mainly because the red mud not only plays the role of physical filling, but also provides the physical filling for hydration reaction of steel slag. The hydration reaction of steel slag provides a good alkaline environment, which promotes participation of steel slag in the hydration reaction and generates more calcium alumina and hydrated calcium silicate gel to improve the microstructure of mortar.