Phosphogypsum-lime-flyash-gravel mixture (PLFG) is usually used to strengthen road foundation of geotechnical engineering. It is important to correctly understand its compressive behavior before corresponding design. In this present paper, triaxial compression test on PLFG was carried out, and its stress-strain curve was investigated. Experiment data show that PLFG is with high bearing capacity and good stiffness. A new composite-exponent model was established, which is available for both hardening and softening type strain-stress curves of PLFG. Finally, comparison between PLFG tested data and new model simulation was performed, and good agreements have been found. This research is helpful for engineering sustainable utilization of phosphogypsum to bring economy and to reduce environmental pollution.