Abstract: The law governing the effect of energy input on hydrophobic aggregation of ultrafine coal particles in high-muddy coal slurry water under the action of 1831 (octadecyl trimethy ammonium chloride) is investigated through measurement of contact angle and zeta potentials, observation of patterns of aggregates formed, and test on sedimentation of aggregates. The interparticle potential with and without addition of 1831 is calculated with extended DLVO theory, and the mechanism underlying the hydrophobic aggregation of fine particles in high-muddy coal slurry water under the effect of 1831 with energy input. Is analyzed as demonstrated by test result, energy input can help promote the effect of 1831 in enhancement of hydrophobicity of particle surface and improve the efficiency of 1831 in reduction of particle surface electronegativity; appropriate energy input can promote the formation of dense hydrophobic aggregates of fine particles, beneficial, therefore to the improvement of slurry settling efficiency; potential energy calculation further confirms that appropriate energy input is the necessary condition for coal slurry to form hydrophobic aggregation under the effect of 1831; and the formation of such aggregates under the action of 1831 is principally attributed to the combined adsorption-charge neutralization and hydrophobic actions, with the latter as the dominant effect.