Abstract: To address the dual challenges of phosphogypsum stockpiling pollution and the high cost of road base materials, this study investigated the mix proportion of phosphogypsum-based foamed concrete with high phosphogypsum content (≥50%). Through the synergistic regulation of water-binder ratio, superplasticizer, and rapid-hardening agent (water-binder ratio of 0.4, superplasticizer dosage of 0.5%, and sulphoaluminate cement content of 6%), the technical bottlenecks of delayed setting, poor slurry stability, and slump tendency in high-volume phosphogypsum systems were resolved. The composition of cementitious materials for foamed concrete was systematically studied. At the optimal mix proportion of phosphogypsum : cement : mineral powder=60:15:25, the foamed concrete achieved a wet density of 800 kg/m³, flowability of 178 mm, 7-day compressive strength of 1.2 MPa, and softening coefficient >0.95, satisfying the comprehensive requirements for flowability, mechanical properties, and water resistance of road base materials. Microstructural analysis revealed that phosphogypsum primarily functions as a micro-aggregate filler in the system, with partial participation in the reaction to generate ettringite frameworks. The C-S-H gel from cement hydration and ettringite synergistically constitute the strength skeleton. At a water-binder ratio of 0.4 with the optimal cementitious material composition, the foamed concrete exhibits spherical pores with uniform distribution and thick pore walls. The hydration products and uniform pore structure serve as the mechanism for achieving enhanced strength and water resistance under high replacement rates.