The invention discloses a hydrophobic surface modification method for improving a flotation effect of oxidized coal slime. The method comprises the following steps: calcium hydroxide is added in water for stirring to form alkaline calcium hydroxide solution; the oxidized coal slime is added in the calcium hydroxide solution to stir for neutralization reaction, and is filtered and dried to prepare an acid-alkali neutralization oxidized coal slime filter cake; organic carboxylic acid is added with water for dissolving, stirring and heating; and the acid-alkali neutralization oxidized coal slime filter cake is added in the organic carboxylic acid to stir at normal temperature for grafting reaction, and is filtered and dried to prepare hydrophobic modified oxidized coal slime filter cake. The method uses a chemical complexing method for grafting hydrophobic chain sections onto acid hydrophilic functional groups on the surfaces of coals, is not easy to be influenced by the flotation solution chemical environment, is firm in combination, preferably improves the problem of weak surface hydrophobicity of the oxidized coal slime, and can improve the flotation and recovery rates of cleaned coals.