The evolution of mineral phase structure during the reduction and melting separation of an rare earth (RE)-rich iron mineral (RER-IM) is investigated. The results show the iron oxides are reduced to their metallic iron or FeO at 1373 K. When reduction time is 180 min, the reduction degree is 84%. Both bastnaesite (RE(CO3)F) and monazite (REPO4) are transformed into Ca2RE8(SiO4)6O2 during carbothermic reduction at 1373 K. The mineral with a reduction degree of 84% is melt-separated in a graphite crucible at 1773 K for 20 min, the resulting slag contains 20.64% RE2O3, with RE existing in the form of Ca2RE8(SiO4)6O2. Moreover, P from the reduction of Ca3(PO4)2 dissolves in iron with a content ranging from 1.2 to 2.21%. The type of RE phase that occurs in the slag is related to the distribution of P between slag and iron. A low P content in the slag facilitates the formation of Ca2RE8(SiO4)6O2, but a high content in the slag favours Ca3RE2[(Si, P)O4]3F. Thus, it is confirmed that the RE phase structure is controlled by the distribution of P between slag and iron.