In order to study the effects of steel slag on the mechanical properties and microstructure of geopolymers, the compressive strength and flexural strength of a fly-ash steel slag–based geopolymer paste/mortar have been explored. Linear analysis of strength of fly-ash steel slag–based geopolymeric mortar with steel slag content was studied. The microstructure and mechanism of fly-ash steel slag–based geopolymer were measured by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and field emission-scanning electron microscope (FE-SEM). The pore structure was tested by the Brunauer-Emmett-Teller (BET) method. The results show that the early strength of geopolymers shows an upward trend with the increase of steel slag content, however, in the later age, the compressive strength of the sample gradually decreases when the steel slag content exceeds 10%. FT-IR and XRD results indicate that both sodium aluminosilicate hydrate (N-A-S-H) gel and calcium aluminosilicate hydrate(C-S-H) gel exist in the fly-ash steel slag–based geopolymer. BET and FE-SEM results show that compared with the control sample (FAP-0), more mesopores and fewer macropores are present in the paste sample with 30% steel slag (FAP-30).