The monazite flotation system often has many associated minerals such as calcite, fluorite, and dolomite. These associated minerals will dissociate a large amount of Ca²⁺ in the monazite ore, and Ca²⁺ often affects the pulp flotation environment. In this paper, the surface properties of monazite were studied by FTIR and XPS when Ca²⁺ affected the flotation of monazite by octyl hydroxamic acid. The research method is mainly through the solution chemical calculation of Ca²⁺, the flotation test of monazite, and the surface infrared spectroscopy ( FTIR ) and X-ray photoelectron spectroscopy ( XPS ) of monazite under flotation conditions. The solution chemical calculation of Ca²⁺ shows that an aqueous solution with increased pH value, Ca²⁺ exists in an ionic state, hydroxyl complex, and hydroxide compound, respectively. At the same time, the dominant components are Ca²⁺ and Ca(OH)⁺ at pH between 7 and 8. The results of monazite flotation showed that OHA could not fully float monazite without adding Ca²⁺, and the recovery rate was 75.37%. When the pH is (8±0.5), and the dosage of Ca Ca²⁺ is 3×10^-4 mol·L^-1, the flotation performance of OHA on monazite can be significantly improved, and the recovery rate reaches 96.48%. According to the solution chemistry calculation, Ca(OH)⁺ is the dominant component of activated monazite. When the Ca²⁺ dose is greater than 3×10^-4 mol·L^-1, the flotation recovery rate decreases greatly, indicating that the further increase of Ca²⁺ dose inhibits the flotation of monazite. Only a certain dose of Ca²⁺ effectively promotes the flotation of monazite, which may be attributed to the consumption of OHA concentration by Ca²⁺ dose, which in turn affects the flotation of monazite. Infrared spectroscopy analysis showed that under the action of Ca²⁺, two key new peaks appeared in the spectrum, one was the N—O—H bending vibration peak at 1 454 cm^-1, the other was the O—N stretching vibration peak at 880 cm^-1, and the organic peaks at 2 974 and 2 928 cm^-1. The —CH₃ and —CH₂— peaks were significantly enhanced, and these groups appeared to indicate that chemical adsorption occurred and the adsorption intensity was greater than that of pure monazite adsorption of OHA. XPS analysis showed that the relative content of N element on the surface of monazite was 0.61 % when only OHA was used to adsorb monazite. In comparison, the relative content of N element on the surface of monazite was 2.36% when OHA was used to adsorb monazite after Ca²⁺ treatment. It can be concluded that Ca²⁺ will promote the adsorption of OHA on the surface of monazite. It can be seen from the peak fitting that the addition of Ca²⁺ reacts with Ca(OH)⁺ to form O—Ca—OH groups on the cleavage surface of monazite to form O—Ca—OH groups, which can be used as a new adsorption site for adsorbing OHA. At the same time, the cerium atom of monazite and the two oxygen atoms on OHA form a five-membered chelate, which also acts as an adsorption site. It is concluded that there can be two adsorption sites on the surface of monazite. The active sites of Ca and Ce atoms on the surface of monazite can adsorb OHA, which is beneficial to the adsorption of OHA on the surface of monazite and forms a more uniform and dense OHA hydrophobic adsorption layer. This is why the performance of Ca²⁺ in OHA flotation of monazite is improved. This study helps to enrich the activation theory of metal ions in pulp and also confirms that effective mineral flotation separation depends not only on the strength of collector-mineral interaction but also on the chemical properties of the flotation solution to a large extent. Utilizing or controlling the surface reaction should be the main goal of developing a more efficient and economical flotation process.