In this paper, steel slag based admixtures (GKF) were taken as the research object. Based on the single factor experiment, the best combination scheme of activator Na2SO4, silicon slag and desulfurized gypsum is investigated by orthogonal experiment. The suitable mass proportion is 2.0%Na2SO4, 0.5%silicon slag and 1.5%desulfurized gypsum, The results show that the activity of the mortar prepared by replacing P·I 42.5 with 50%GKF is 77.3%, 85.9% and 96.6% at 3 d, 7 d and 28 d, respectively. Compared with the group without activator, the activity increases by 24.2%, 25.4% and 22.4%, respectively. XRD and SEM were used to analyze the structure and micro morphology of the hydrated minerals. The results show that the composite activator is helpful to the hydration of GKF, which increases the content of C-S-H and AFt, and makes the structure more compact.