The Poor Flexural and Damping Properties of Building Materials Damages Concrete Structures and Affects their Service Life When Concrete Structures Are Subjected to Dynamic Loads. Three Different Dosages (I.e., 0%, 0.3%, and 0.6%) of Organic Phosphonates (HEDP.4Na) and Different Pouring Methods (I.e., Conventional Pouring Method, 90°-Induced Pouring Method, and 150°-Induced Pouring Method) Were Designed to Improve the Flexural and Damping Performance of Fiber-Reinforced Alkali-Activated Slag Composites (FR-AASC). the Enhanced Mechanism of HEDP.4Na Was Revealed by Phase Analysis (X-Ray Diffraction, XRD), Pore Structure Analysis (Mercury Intrusion Porosimetry, MIP), the Heat of Hydration, and Scanning Electron Microscopy (SEM) Analysis. the Results Showed that 0.3% HEDP.4Na Combined with the 150°-Induced Pouring Angle Can Significantly Improve the Mechanical Properties of the FR-AASC Sample Compared with the Reference Group. the Sample with 0.3% HEDP.4Na Cast by the 150°-Induced Pouring Angle Increased Compressive and Flexural Strength, Damping Energy Consumption and Storage Modulus by 20%, 60%, 78%, and 30%, Respectively, Compared with the Reference Sample Cast by the Conventional Pouring Methodology. HEDP.4Na Reduced the Early Hydration Heat and Total Porosity of the FR-AASC Matrix, Modified the Fiber–matrix Interface Transition Zone, and Increased the Frictional Energy Consumption of Steel Fibers. overall, the Synergistic Effect of HEDP.4Na and the Induced Pouring Methodology Significantly Improved the Flexural and Damping Properties of FR-AASC. This Study Can Provide a Guidance for Improving the Flexural and Damping Capacity of FR-AASC and Promote the Application of FR-AASC in Construction Engineering.