The rapid setting characteristic of alkali-activated cementitious materials ( AACM) has restricted its wide application in engineering, especially at high alkali dosage. In this study, an industrial by-product desulfurization gypsum was introduced into AACM to solve this problem. Meanwhile, mechanical properties and hydration reaction characteristics were investigated to study the effect of desulfurization gypsum on the properties of AACM, and microscopic analysis methods such as XRD and SEM were used to explore the action mechanism of desulfurization gypsum on AACM. The results indicate that the incorporation of desulfurization gypsum can effectively prolong the setting time of AACM, and that the content of desulfurization gypsum is positively correlated with the setting time, but negatively correlated with the strength, hydration heat flow and total heat, the content of desulfurization gypsum should not exceed 9% (mass fraction) of the cementitious materials. The incorporation of desulfurization gypsum does not produce a large number of ettringite crystals in matrix of AACM, however, the microstructure is loosened which resulted in a decrease in strength.