Compaction test， unconfined compressive strength test and California bearing ratio test were conducted to modified phosphogypsum. The settlement of roadbed with different compaction degree and height （modified phosphogypsum system as roadbed filler） was simulated by geotechnical centrifuge. The final settlement of subgrade with different boundary conditions was obtained by compression test and layer-summation method. The optimum mix ratio after comprehensive consideration is semi-hydrated gypsum accounts for 10% of the mass of air-dried phosphogypsum， liquid adhesive accounts for 0.5%.The modified phosphogypsum satisfies strength requirement of roadbed. Embankment（20 m height and below）with a compaction degree of 85%、95% and 100% satisfies the post-construction settlement requirements of 20 cm.