Abstract: This study aimed to elucidate the influence of thermal decomposition under an inert atmosphere on the phase composition, microstructure, and flotation performance of bastnaesite. Experiments showed that decomposition was strongly temperature-dependent. After complete decomposition, the release of CO₂ increased the rare earth oxide grade from 72.90wt% to 86.83wt%, accompanied by an increase in the Ce oxidation degree. Major decomposition products included rare earth oxyfluoride (REOF), rare earth trifluoride (REF₃), and Ce₇O₁₂, with some products showing low crystallinity. The decomposition damaged the particle structure, resulting in the extensive lamellar cracking, a significant increase in specific surface area, and partial fragmentation of the particles. Flotation tests revealed that optimum recovery was achieved at pH 8.00–9.00. However, thermal decomposition increased the initial pulp pH to almost 11.00, making pH adjustment difficult. Salicylhydroxamic acid (SHA) was adsorbed on the surfaces by both physical and chemical interactions, with chemical adsorption being significantly enhanced after decomposition. During flotation, SHA was distributed not only on particle surfaces but also in internal pores after decomposition. Due to the phase and microstructural changes, the required dosage of SHA increased from 20 mg·L⁻¹ for the raw ore to 250 mg·L⁻¹. These results provide insights into the development of reagents suitable for the flotation of bastnaesite roasting products.