The invention discloses a method for recovering polyether from waste metal processing liquid and application of the polyether in concrete slump retaining agent. The method for recovering the polyether from the waste metal processing liquid comprises the following steps: adding inorganic base to water-based waste metal processing liquid containing the polyether to adjust the pH value of the waste metal processing liquid to 10-12, standing the waste metal processing liquid for reaction, filtering, adding activated carbon and an inorganic acid to adjust the pH value of the waste metal processing liquid, heating to the temperature above the cloud point temperature, layering solution, standing at constant temperature, taking polyether phase on the upper layer, adding anti-oxidant to the polyether phase, and fully drying to remove water until the water content is 0.05 percent by weight or less to obtain the reaction product which is polyether. The following raw materials are needed when the polyether is applied to the concrete slump retaining agent: a catalyst, maleic anhydride, water, sulfonated styrene and an initiator in addition to the polyether. The polyether and the maleic anhydride are directly subjected to the esterification reaction under the heating condition without adding organic solvent and water and adopting a condensing and refluxing device, thus the equipment requirement and the production cost are reduced, and the production safety is improved. The prepared concrete slump retaining agent has good slump retaining performance, high water reducing rate and good compatibility with cement.