Abstract: The massive accumulation of chloride-containing titanium extraction tailings (CCTET) and boron mud (BM) poses a serious ecological risk, underscoring the need for synergistic valorization strategies. An integrated approach is proposed: co-blending CCTET and BM with light-burned MgO (LBMO) to produce fiber-reinforced magnesium oxychloride cement (MOC) boards. Optimal dosages were identified as 21.2 wt% CCTET and 15.9 wt% BM, at which the composite shows pronounced performance gains. The resulting boards deliver 28 d flexural strength of 12.08 MPa, a softening coefficient of 0.86, chloride-ion leaching of 2.96%, and a density of 1.69 g cm⁻³, meeting technical specifications for high-performance building materials. Adding 1 wt% polyethylene fibers, the flexural strength of the board increases further to 15.48 MPa, and the softening coefficient rises to 0.9. Relative to conventional MOC boards, the material exhibits 130.7% higher flexural strength, 70% higher softening coefficient, and an immobilization efficiency of 68% for chloride ions. Mechanistically, synergy between CCTET and BM promotes formation of 5Mg(OH)₂·MgCl₂·8H₂O (Phase 5) and stable Mg-Cl-Si-Al-H/Mg-Cl-Si-H gels, which enhance mechanical stability and water resistance while suppressing chloride-ion leaching. This work provides a promising route to upcycle industrial solid wastes into sustainable, high-performance building materials.