Abstract: A method for treating waste water is provided, in which a wastewater is treated through a distilling process to obtain a distilled liquid, and then a reactant is added into the distilled liquid during the distilled liquid flowing to a liquid collecting tank, such that the reactant reacts with the distilled liquid by utilizing the residual temperature of the distilled liquid to form a processed liquid. Meanwhile, a system for wastewater treatment is also provided, which includes a distillation unit, a guiding unit, a means for adding the reactant, and a collecting unit. Since the present invention utilizes the residual temperature of the distilled liquid for the need of the reaction, an additional heat source is unnecessary, such that the consumption of energy can be greatly reduced.