Aiming at the problem that the floatability of coal slime becomes worse after oxidation and has adverse effect on the flotation effect, the effect of oxidation degree on the floatability of coal slime and its mechanism are studied by using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, spectrophotometer, contact Angle test, reagent adsorption measurement, unit flotation and step release experiment. The results show that for Suhaitu 9#, Guoyutela coal and Lutian 9# coal with oxidation degree of 21.10%, 48.30% and 53.25%, the higher the oxidation degree, the more functional groups such as -OH and >C=O and hydrophilic alumino-silicon minerals containing oxygen, and the less hydrophobic groups such as -CH3 and -CH2. The higher the proportion of oxidized part on coal surface, the more dominant the hydrophilic groups on coal surface, and the higher the hydrophilicity of coal, the worse the slime flotation effect. At the same time, the oxidation degree of coal increases, resulting in the reduction of the contact Angle of coal, the deterioration of surface hydrophobicity and the deterioration of diesel oil adsorption capacity. When the ash content of cleaned coal is 10%, the recovery rate of the coal with moderate oxidation can be increased from 11.81% to 43.54% compared with only using diesel.