Slow-release phosphate materials were prepared by activating insoluble phosphate with organic acid to stabilize high concentrations of Cd and Zn in contaminated smelter soil. The results showed that oxalic acid (0.1 mol/L) activated tricalcium phosphate (TO-0.1) provided the most efficient stabilization of Cd and Zn. After 30 d treatment, leaching concentrations of Cd and Zn in soil were decreased from 3.17 and 16.60 mg/L to 0.078 and 0.32 mg/L, respectively. The acid-soluble fractions of Cd and Zn were transformed into reducible, oxidizable, and residual fractions. Notably, As mobility in TO-0.1 treated soils did not increase. In addition, acid rain leaching and 150 d of natural aging revealed that the slow-release phosphate material provided long-term stability for the stabilization of Cd and Zn. This study verifies the potential application of slow-release phosphate materials for the remediation of heavy metal contaminated soil at smelting sites.