This study employs phosphogypsum as the primary infill in the preparation of FRP-confined phosphogypsum-based geopolymer. Through compressive strength experiments of 21 specimens, the effects of FRP layers, alkali activator dosage, phosphogypsum dosage, fly ash dosage, and calcium content in fly ash on the compressive strength and stress-strain relationship of the specimens were investigated. The results indicated that specimens constrained by a single FRP layer exhibited a compressive strength approximately 6 times that of unconstrained specimens, while those constrained by two layers showed a 1.5-fold increase over the single-layer specimens. The substantial enhancement in strength of the constrained specimens is attributed not only to the excellent mechanical properties of FRP but also to the slightly expansive characteristics of gypsum, which aid in increasing the strength of the constrained components. Furthermore, the addition of constraints reduced the exposure of the geopolymer to wet-dry cycling and carbonation, thereby suppressing the efflorescence issues and preventing strength loss in the geopolymer. Additionally, incorporating a small amount of high-calcium fly ash into the infill further enhanced the compressive strength of the composite components. This research provides a novel approach to the reuse of phosphogypsum.