Abstract: To address the engineering requirements of utilizing phosphogypsum as the sole filling aggregate in mine backfilling, the strength development patterns and microscopic mechanisms of phosphogypsum cemented backfill over curing periods of 7−90 d were investigated through laboratory mechanical testing combined with microscopic analysis techniques including X−ray diffraction, thermogravimetric differential scanning calorimetry and scanning electron microscopy. This integrated methodology enabled detailed analysis of hydration product evolution, microstructural characteristics, and their correlation with macro−mechanical properties. Research results indicate that the compressive strength of phosphogypsum backfill increases linearly with curing age during the 7−56 d curing period, exhibiting notably low early−stage strength. The 28 d strength accounts for only 40%~50% of 90 d strength, demonstrating a “slow initial setting followed by continuous growth in the middle and later stages” development pattern. The strength development is governed by hydration product accumulation and microstructural evolution. At the age of 7 d, gypsum and soluble phosphorus, fluorine inhibit cement hydration, resulting in AFt being distributed in fine needles and a small amount of C−S−H gel formation. This results in high porosity and low early strength of phosphogypsum cemented backfill. At the age of 28 d, sustained hydration of cement minerals (C₃S, C₂S) promotes the growth of AFt crystals from fine needles to needle rods, interweaving with C−S−H gel to form a network skeleton, reducing porosity. At the age of 56−90 d, C₃S is nearly depleted, and C−S−H gel densely encapsulates AFt and dihydrate gypsum crystals, while recrystallized dihydrate gypsum refines and fills pores, forming a compact structure that significantly enhances the backfill’s compressive strength. This study reveals the retardation mechanism and structural densification process of phosphogypsum cemented backfill at the microscale, providing crucial technical support for the large−scale application of phosphogypsum in mine backfill engineering.