This study investigates the structural damage evolution and mechanical degradation of epoxy-modified magnetic cementitious slurry under freeze-thaw cycles. To elucidate the damage mechanism, slurries with varying magnetic powder contents were subjected to freeze-thaw testing. The evolution of pore distribution was quantitatively analyzed using Computed Tomography (CT) and Scanning Electron Microscopy (SEM) via image binarization. The results reveal that: (1) An increase in magnetic powder content significantly mitigated structural damage, indicating that magnetic particles positively contribute to the freeze-thaw resistance of the slurry; (2) The application of an external magnetic field significantly enhanced frost resistance, with magnetically aligned specimens exhibiting reduced damage levels post-cycling compared to non-magnetized controls; (3) Under identical magnetic powder contents, specimens subjected to a magnetic field demonstrated notably higher compressive strength and a significantly lower strength loss rate after freeze-thaw cycles compared to those without magnetic field treatment; (4) As the magnetic powder content increased, the porosity within the specimens gradually decreased, resulting in a more compact structure and improved freeze-thaw resistance; (5) Magnetic particles self-assembled into four primary chain morphologies: through, branched, isolated, and bundled chains. The evolution of these morphologies plays a critical role in governing the mechanical properties and structural stability of the composite.