At present, methods of separating from low-grade barite ore are usually adopted to improve the grade of barite in flotation by using new flotation reagents, and the recovery of flotation can be improved by microwave pretreatment of the mineral before flotation, but the mechanism of microwave on flotation reagents and minerals are not yet known. In recent years, microwave heating technology has been used in mineral processing, metallurgy and material preparation and so on, with the advantage of fast reaction speed and high product index. In this paper, sodium oleate was used as a collector in the flotation tests of the pure barite mineral after microwave pretreatment, and the infrared spectrum detection was carried out for the barite flotation samples under different microwave heating times. The effect mechanism of microwave on barite flotation was discussed by the infrared spectral analysis of fitting smooth spectrum and second derivative spectrum. The flotation test results showed that the barite without microwave pretreatment had the best flotation index and recovery of 91.41% under the conditions of sodium oleate dosage of 55 mg·L^-1 and pH value of 8.0. In contrast, the flotation index of barite treated with microwave increased gradually with the increase of microwave treatment time, and the recovery rate of barite treated with microwave in the 60 s was the highest, reaching 95.27%. Infrared spectrum analysis based on flotation test showed that in the flotation of barite without microwave pretreatment interacted with sodium oleate, the red shift of —CH₂— symmetric stretching vibration peak at wavenumber 3 004 cm^-1, —CH₃ antisymmetric stretching vibration peak at wavenumber 2 953 cm^-1, $SO^{2}_{4}$asymmetric stretching vibration peak at wavenumber 1 119 and 1 077 cm^-1 were all found, indicating chemical adsorption of sodium oleate on the surface of barite was happened; By contrast, in the flotation of barite after microwave pretreatment interacted with sodium oleate, the red shift of —CH₂— symmetric stretching vibration peak at wavenumber 2 853 cm^-1, —CH₂— symmetric stretching vibration peak at wavenumber 2 923 cm^-1, —CH₃ antisymmetric stretching vibration peak at wavenumber 2 958 cm^-1, $SO^{2}_{4}$ asymmetric stretching vibration peak at wavenumber 1 181, 1 122 and 1 086 cm^-1, $SO^{2}_{4}$ symmetric stretching vibration peak at wavenumber 982 cm^-1, $SO^{2}_{4}$ bending vibration peak at wavenumber 635 and 610 cm^-1 were not found, but the peak strength increased significantly with the microwave heating time, and the peak strength increased most significantly with the microwave heating time of 60 s. The fitting smoothing spectrum and the second derivative spectrum calculations of the infrared spectrum after microwave pretreatment showed that the peak area at wavenumber 2 958, 2 923, 2 853, 1 181, 1 122, 1 086, 982, 635 and 610 cm^-1 increased to different degrees, and under microwave heating 60 s, the peak area respectively increased by 1.84%, 259.12%, 761.15%, 235.72%, 145.61%, 198.50%, 641.16%, 549.67% and 744.97%. It indicates that microwave pretreatment does not induce a chemical reaction on the barite surface, but strengthens the chemical adsorption between sodium oleate collector and barite ore, make the chemical adsorption on the barite surface denser, and the adsorption quantity increased, so the barite recovery increases and the flotation index improves.