Abstract: The surface oxygen content of low rank coal is high and the surface hydrophobicity is poor. It is difficult for diesel oil to adsorb on the coal surface efficiently. The reagent consumption is large and the flotation effect is not ideal. In this paper, diesel oil and carboxylic acid of oleic acid, lauric acid, decanoic acid and octanoic acid were combined to carry out flotation test on Datong Jurassic low rank coal. The results show that the addition of oleic acid, lauric acid and decanoic acid can effectively improve the flotation effect, and oleic acid and lauric acid have the most obvious effect. Compared with diesel oil alone, the oil rate can reach 50%. In addition, the addition of oleic acid and lauric acid can effectively increase the contact angle of coal sample surface and make the coal sample more hydrophobic. The effect of n-decanoic acid is weaker than that of oleic acid and lauric acid. Among the four carboxylic acids with 10% mass fraction and diesel oil, oleic acid has the smallest particle size in slurry, followed by lauric acid; and through XPS analysis, it is found that oleic acid can effectively increase the C-C, C-H hydrophobic oxygen-containing functional groups of coal samples, and reduce the C-O hydrophilic oxygen-containing functional groups. The dispersion of the reagent is improved effectively, and the hydrophilicity of the coal sample is weakened, and the hydrophobicity is enhanced, which improves the flotation effect.