To solve the problem of high energy consumption in tailings pretreatment methods and explore a low energy consumption tailings activation method, alkaline hydrothermal activation pretreatment was carried out on part of sulfur tailings to enhance their activity and improve the mechanical properties of sulfur tailings-based geopolymers. The influences of activation temperature and activation time on alkaline hydrothermal activation effect have been studied. X-ray diffraction technology, Fourier transform infrared spectroscopy analysis, thermogravimetric analysis, nuclear magnetic resonance imaging analysis, and scanning electron microscopy test were used to characterize the performance of activated sulfur tailings and sulfur tailings-based geopolymers. The results show that when the sulfur tailings are activated at 200 ℃ for 2.0 h, the prepared geopolymer obtains the highest 3 and 28 d compressive strength, which are 15.5 and 22.1 MPa, respectively. Alkaline hydrothermal activation can effectively destroy the crystalline phase in sulfur tailings, promote the release of active silica and aluminum. More hydration product gel is formed in the gel system to improve the pore structure of geopolymer, thus improving the compressive strength of sulfur tailings-based geopolymers. The above experimental results indicate that alkaline hydrothermal activation is a method that can effectively improve the activity of sulfur tailings.