The invention relates to a method for preparing a coal slime based carbon-silicon dioxide composite reinforcing agent by using coal slime as raw materials. The method comprises the steps of (1) putting the coal slime in an N2 atmosphere for calcination, so that carbonized coal slime is obtained; (2) adding the carbonized coal slime to a hydrochloric acid solution with the concentration of 10% to 25%, performing acid leaching and stirring, and then drawing and filtering and washing to be neutral with distilled water, so that acid leaching coal slime residues are obtained; (3) adding a NaOH solution with the concentration of 10% to 15% to the acid leaching coal slime residues, and stirring to react for 2 h to 3 h at the temperature of 80 DEG C to 100 DEG C; (4) adding the hydrochloric acid solution with the concentration of 10% to reactant obtained in the step (3) at the temperature of 40 DEG C to 60 DEG C, adjusting the pH to 5.0 to 6.0, stirring to react, and then drawing and filtering and washing to be neutral with distilled water, so that a coal slime based carbon-silicic acid compound is obtained; (5) calcining the coal slime based carbon-silicic acid compound at low temperature, so that a coal slime based carbon-silicon dioxide compound is obtained; (6) adding modifiers to the coal slime based carbon-silicon dioxide compound obtained in the step (5), and performing ball-milling treatment, so that the modified coal slime based carbon-silicon dioxide composite reinforcing agent is obtained.