The invention discloses a method for extracting ultra-low-ash-content pure coal from coal slime. The method comprises the following steps: ball-milling slurrying, specifically, milling coal slime until particles with less than 200 meshes occupy more than 90%, and controlling the mass percent concentration of the coal slime slurry to be 10-35%; grading, specifically, conveying the particles with less than 200 meshes to a flotation step; flotation, specifically, adding a collecting agent, a foaming agent and an inhibitor in the obtained coal slime slurry, uniformly mixing, and carrying out one-time rough flotation, two sections of three-to-six-time fine flotation and one section of three-to-six-time scavenging to obtain refined coal slurry I, refined coal slurry II and tailing coal slurry respectively; and dehydration, specifically, adding a flocculant in the refined coal slurry I, the refined coal slurry II and the tailing coal slurry which are obtained through the flotation, carrying out flocculation settling, then dehydrating by virtue of an ultrahigh-pressure pressure filter, and controlling a pressure filter pressure to be 4.5-5.5Mpa, thus obtaining ultra-low-ash-content pure coal products, common refined coal products and tailing coal products. The method disclosed by the invention is simple in process flow, and capable of efficiently extracting the ultra-low-ash-content pure coal products from the coal slime; and moreover, the water contents of the obtained coal products are low and can achieve less than 15%, thus the coal products have remarkable economic benefits and environmental protection value.