To investigate the impact of carbonation on the mesostructure and mechanical properties of dam concrete, scanning electron microscope (SEM), X-ray computed tomography (X-CT), and combined uniaxial compression-acoustic emission tests were conducted on concrete specimens from the Meishan Arch Dam. Based on Avizo images and acoustic emission parameters, the mesostructural characteristics (including porosity, pore size distribution, and pore morphology) and material damage evolution characteristics of carbonated dam concrete were analyzed. The results show that carbonation alters the microscopic morphology of concrete, numerous calcium carbonate particles appear on the surface of the C-S-H gel, and the porosity decreases. As carbonation progresses, the proportion of pores with equivalent diameters larger than 160.μm increases, while the proportion of pores with equivalent diameters lower than 160.μm decreases. After 14-day and 28-day accelerated carbonation, pores with a sphericity of 0.9 to 1.0 increase by 8.9% and 6.5%, respectively, compared with that of the non-carbonated concrete, indicating that carbonation fills the irregular edges of pores. After carbonation, the overall activity of acoustic emission signals of dam concrete decreases, and the compressive strength and elastic modulus of concrete decline.