Alkali-activated cementitious materials (AAM) are regarded as a sustainable alternative to ordinary Portland cement (OPC). The incorporation of suitable inert supplementary materials in AAM can modulate properties, reduce production costs and energy consumption, facilitate wider application and commercialization of the AAM, and ultimately contributes to lowering carbon emissions in the construction sector. Although deserts contain abundant desert sand, the fine particle size and poor gradation of desert sand significantly limit its large-scale engineering applications. To expand the application of desert sand in the engineering field, this study uses untreated desert sand (DS), temperature-treated desert sand (DS-T), desert sand ground for 20 min (DS-G20), and desert sand ground for 30 min (DS-G30) as inert supplementary materials to prepare alkali-activated ternary cementitious material pastes. The study systematically investigates the effects of desert sand treatment methods and content on the hydration performance, drying shrinkage, pore structure, and strength of alkali-activated ternary cementitious materials with ground granulated blast furnace slag and fly ash. The study finds that four types of desert sand primarily serve as skeleton fillers in pastes. The 28-day flexural strength and total cumulative pore volume of pastes with desert sand are higher than that of the control paste. As desert sand content increases, the hydration exothermic rate of the pastes decreases, while the diffraction peaks intensity of quartz, drying shrinkage, and mass loss of the pastes increase. DS-G30 optimizes the particle grading of the mixed powders, which improves the pore structure of pastes, regulates the hydration reaction of pastes, and reduces the drying shrinkage of pastes. Therefore, DS-G30 demonstrates the most optimal improvement effect on alkali-activated ternary cementitious materials. This study can provide technical support for the application of desert sand in cementitious materials, while the use of desert sand can reduce the cost and production energy consumption of AAM.