To provide basic research for the development and application of advanced technology for the metallurgy of Bayan Obo mixed rare earth concentrate (BOMREC), the thermal decomposition behavior of BOMREC under nitrogen atmosphere was investigated by using the Kissinger method, TGA-DSC and XRD etc., including the thermal decomposition kinetics, phase change, cerium oxidation efficiency, and phase change effect on the leaching efficiency of rare earth elements (REEs). The results indicate that during the roasting at 500-550 °C, the mass loss and activation energy (E_a) are approximately 10% and 148 kJ/mol, respectively. After roasting at 550 °C for 2 h, the bastnaesite in the BOMREC is completely decomposed and converted into rare earth oxides and fluorine oxides. The maximum oxidation efficiency of cerium reaches 0.58%. The maximum leaching efficiency of REEs reaches 49.1% after thermal decomposition at 600 °C for 2 h.