The pyrolysis separation of calcium and magnesium from phosphate tailings is an important process due to its high-value resource utilization. In this paper, aiming to address the problems of high energy consumption, a slow decomposition rate and the low activity of decomposition products in the high-temperature pyrolysis of phosphate tailings, the medium-temperature pyrolysis of phosphate tailings under a H₂O atmosphere was carried out, and the phase reconstruction and activation of pyrolysis process were discussed. The results showed that compared with N₂, air and CO₂ atmospheres, the pyrolysis process of phosphate tailings in a H₂O atmosphere was changed from two stages to one stage, the starting decomposition temperature was reduced to 500 °C and the decomposition time was shortened to 30 min. The order of the influence of each factor on the pyrolysis of phosphate tailings was temperature > H₂O pressure > holding time. Under the optimized pyrolysis conditions, the yield of CaMg(CO₃)₂ decomposition of phosphate tailings into MgO and CaO was 97.3% and 98.1%, respectively, and the reactivity of MgO was 31.6%. The distribution of Ca and Mg elements in the phosphate tailings after pyrolysis showed a negative correlation, and both of them no longer formed associated compounds; Ca mainly existed in the form of Ca(OH)₂, Ca₅(PO₄)₃F, CaSiO₃ and CaF₂, and Mg mainly existed in the form of MgO, MgF₂ and Mg(OH)₂.