Abstract: To investigate the swelling properties and microstructure evolution of compacted bentonite in an annular technological void under different temperatures, swelling pressure tests are conducted on compacted bentonite in an annular technological void. After technological void is closed, water content, dry density and microstructures is determined, respectively. Results show that the swelling pressure time-history curve shows a single-peak pattern, initially rising to the peak before falling and stabilizing. The dynamic equilibrium of the "wedge" force, formed by the thickening of bound water film, laminar cleavage, pore collapse and thickening of diffusion double layers, dominates the force states from the vertical lateral limit to the constant volume. Higher temperatures intensify the increase water content and decrease dry density from the interior to the exterior. The swelling behavior is enhanced by the increase in montmorillonite expansive coefficient and water molecule diffusive coefficient with temperature, leading to more inter-assemblage pores being occupied by the swollen matrix. At higher temperatures, more water molecules entered the interlayers, causing higher hydration reactions, lamellar cleavage, and pore collapse, which reduced the total void ratio and enhanced the swelling properties.