To investigate secondary pollution issued during the preparation of sintered brick from waterquenched yellow phosphorus slag, the composition of slag was experimentally measured in this study. The thermal conversion process and gas phase products associated with S-, P-, F-, and As-containing species present in the heating system were theoretically calculated by means of thermochemical software FactSage 7.0 and databases. The results showed that F and As were released at 700ºC and the gaseous products contained AsF₃. Also, large amounts of F remained in solid CaF₂ and Ca₁₀(PO₄)₆F₂. At a calcination temperature of 900ºC, all As was transferred into gaseous AsF₃ and S started to convert into gaseous SO₂ and SO₃. Other data suggested that the released amounts of SO₂ and SO₃ increased as calcination temperature rose. At calcination temperatures ranging from 100-1000ºC, all P existed as solid Ca₁₀(PO₄)₆F₂. These findings indicated that low calcination temperatures were beneficial for reducing released harmful gases during the production of sintered brick.