In this paper, the effect of shrinkage reducing agent (SRA) on the mechanical property and drying shrinkage of alkali-activated coal gangue-slag (AACGS) cementitious materials was investigated. And the variation of hydration heat, hydration products and microstructure with incorporating SRA was further discussed. The results show that incorporation of 3% （mass fraction） SRA significantly reduces the mechanical property of AACGS cementitious materials, but the reduction amplitude gradually decreases with the progress of hydration. The drying shrinkage decreases significantly with the increase of SRA content. Microscopic analysis shows that the addition of SRA does not lead to the formation of new hydration products, but delayes the hydration reaction, inhibites the early polymerization of alkali-activated materials and increases the porosity of alkali-activated cementious materials, which causes the decrease of mechanical property of AACGS cementious materials with SRA. In addition, SRA does not significantly change the concentration of alkali metal ions in pore solution. The delay of hydration process and the formation of solid phase structure caused by the addition of SRA are the main reasons for reducing the drying shrinkage of AACGS mortar.