The invention discloses a method for preparing a polycarboxylic acid water-coal-slurry additive by using low-rank coal hydrothermal treatment waste liquid. The method comprises the following steps: (1) preparing polyethylene glycol single acetate by carrying out reaction between polyethylene glycol and acetic acid, and then adding acrylic acid and/or maleic acid for esterification to obtain a pre-reaction system; (2) dispersing the prepared pre-reaction system, sodium vinyl sulfonate and/or sodium p-styrenesulfonate and any one of acrylic acid and/or maleic acid into the low-rank coal hydrothermal treatment waste liquid, and carrying out polymerization in the presence of a chain transfer agent and an initiator; and (3) adjusting the pH value of the reaction system prepared in the step (2) to 9 to 11 so as to obtain the polycarboxylic acid water-coal-slurry additive. Compared with a domestic representative commodity additive, the additive disclosed by the invention enables the pulping concentration of low-rank coal such as lignite to be increased by 0.94 to 1.15 percent.