Steel slag tailings are the main solid waste in metallurgical industry, with the production of 15%~20% of crude steel. The utilization ratio is quite low and only reaches 10% of steel slag tailings production due to limited technology. Meanwhile, steel slag tailings are disposed by direct stacking and landfill in general since the management system is not perfect, which pollutes land source, underground water source and air quality. Recycling of solid waste is one important method to achieve sustainable development of resources. The main chemical composition, i. e., CaO, SiO2, Al2O3, MgO, Fe2O3, MnO, f-CaO, etc. and mineral composition, i. e., tricalcium silicate, dicalcium silicate, monticellite, dicalcium ferrite, etc. of steel slag tailings are almost similar to that of cement clinker, making them as the cementitious material with potential cementitious activity. In this paper, steel slag tailings were stimulated physically by mechanical grinding to obtain powder with various sizes. Then, mortar samples of steel slag tailings were prepared in light of Steel slag powder used for cement and concrete (GB/T 20491—2006) and Method of testing cements-Determination of strength (ISO) (GB/T 17671—1999). Then, the impact of steel slag tailings powder at different size, i. e., A40, A60, A80, A100 and A120, on cementitious activity was investigated experimentally, as well as the impact of hydration time on cementitious activity, namely at 3, 7 and 28 d. After that, the size distribution of steel slag tailings powder was characterized by LPSA, and mineral composition and steel slag tailings mortar were tested by XRD and microstructure of steel slag tailings mortar was tested by SEM in order to obtain physical excitation mechanism of steel slag tailings. The results showed that the cementitious activity of steel slag tailings powder presented an increasing trend followed by a decreasing trend as size of steel slag tailings powder decreased. When grinding time was 80 min, the maximum activity occurred at A80. The activity index showed as 67.55%, 71.96% and 73.61% for 3d, 7 and 28 d respectively. As the powder size decreased, the characteristic peak of RO phase in XRD analysis kept a steady intensity while characteristic peak intensity of Ca2SiO4 and Ca3SiO5 increased first but decreased afterwards. The hydration reaction of Ca3SiO5 and Ca2SiO4 could generate Ca(OH)2 and C-S-H gel with good cementitious activity. Small amount of Ca2SiO4 could be found in A80 steel slag tailings powder while large amount of Ca3SiO5 existed, both of which could be used to generate Ca(OH)2 and C-S-H gel. They can improve the mechanical property of A80 steel slag tailings mortar slightly at early period of 3~7 d but greatly at later period of 7~28 d. When hydration time reached 3 d, small amount of hydration products and many micro particles existed in A80 steel slag tailings mortar. For 7 d, the abovementioned hydration products increased to a great extent and larger particles formed. At 28 d, lots of hydration products were generated and there was rarely dispersed micro particles. Under this condition, recycling of solid wastes could be accomplished so as to increase benefit and would not pollute environment severely any more.