The invention relates to a modification method for treating coal ashes of power plant wastewater. The method comprises the steps of (1) screening out the most suitable modifier, wherein the modifier comprises 0.1g of sodium chloride, 0.3ml of 1mol/L sulfuric acid and 0.3ml of 1mol/L hydrochloric acid on the basis of 1g of coal ashes; and (2) modifying the coal ashes, namely mixing and stirring the mixed coal ashes and the modifier at room temperature for 30min, filtering, repeatedly washing by using deionized water, carrying out sufficient electrothermal drying at 120 DEG C, and grinding to prepare the modified coal ashes; and the application method of the modified coal ashes comprises the steps of fully stirring and mixing wastewater and the modified coal ashes in the ratio of 10:1, and controlling the adsorption time to be 45-60min. The modified coal ashes have extremely high adsorption capacity on the power plant wastewater, and the defect of high cost of original power plant wastewater treatment through activated carbon is avoided, thereby achieving the target of treating wastes by using wastes.