In order to explore the evolution characteristics of the meso-structure of slag solidified soil under load, this paper carried out in-situ computed tomography (CT) scanning test during uniaxial compression. The quantitative information of fractures in 2D and 3D form was extracted by combining image processing and 3D reconstruction techniques. With the volume parameters of fractures, the structural damage variables of slag solidified soil were calculated accordingly. Subsequently, a microscopic damage model was established to quantitatively describe the stress-strain relationship of the samples. The results show that the stress-strain curves of slag solidified soil under uniaxial compression loading obviously presents characteristics strain-softening, with a peak stress of 1.85 MPa. With the increase of axial strain, the 2D porosity and the dispersion of fracture distribution increase continuously. The 3D reconstruction model dynamically reveals the cracking process and the evolution of the main fracture surface, and the 3D porosity increases from an initial 3.39% to 9.78% at failure. Both the 3D porosity and fracture connectivity exhibit an logarithmic function relationship with axial strain. The failure of the sample involves three distinct stages: fracture initiation (ε=0%~0.2%), rapid propagation (ε=0.2%~1.5%) and stabilization (ε=1.5%~2.0%). The mesoscopic damage model based on damage variables, which were deprived from fracture volumes, is simple in form, easy to determine parameters, and can accurately predict the stress-strain curves of slag solidified soil. This study provides a novel perspective for the multi-scale analysis of failure mechanism of slag solidified soil.