Abstract: To promote the high-value resource utilization of polymerized aluminum chloride (PAC) waste residue and expand the application scope of magnesium oxychloride cement (MOC), this study developed a high-toughness magnesium oxychloride cement engineering cementitious composite (MOC-ECC) containing PAC waste residue. By conducting water resistance tests, uniaxial compression and tensile tests, combined with X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses, the effects and underlying mechanisms of PE fibers and PAC waste residue on the mechanical properties of MOC were systematically revealed. The results indicate that PAC waste residue significantly improves the water resistance of MOC. The synergistic effect of PE fibers and PAC waste residue effectively enhances the mechanical properties of MOC, with the compressive strength of MOC-ECC reaching up to 84.00 MPa, an increase of 38.61% compared to plain MOC. At a fiber content of 2.0%, the material retained a residual stress of 15.50–20.00 MPa at a strain of 3.0%, demonstrating excellent ductility. Based on this, a uniaxial compressive constitutive relationship model for MOC-ECC was established to characterize the stress–strain mechanical response. In addition, the incorporation of PE fibers into the magnesium oxychloride cement composite (MOCC) induced a strain hardening phenomenon in the uniaxial tensile stress-strain curve. Although the addition of PAC waste residue slightly reduced the tensile strength, it contributed to a significant improvement in the material’s toughness. The MOC-ECC containing 2.0% PE fibers and 20% PAC waste residue achieved an ultimate tensile strength of 8.53 MPa and a tensile strain of 5.50%, representing increases of 46.06% and 223.53%, respectively, compared to plain MOC. The MOC-ECC developed in this study combines excellent mechanical performance with the high-value utilization of waste residue, effectively expanding the application potential of MOC and demonstrating promising engineering applicability.