A stable dispersant and an application thereof in preparing copolymer polyols, the preparation method for the stable dispersant including the steps of 1) contacting a polyol with a dianhydride compound for reaction so as to prepare an adduct; 2) performing a ring-opening addition reaction on the adduct obtained in step 1) and an epoxy compound to prepare a stable dispersant; the dianhydride compound does not contain a double bond that may copolymerize with an olefinically unsaturated monomer, while the epoxy compound contains a double bond that may copolymerize with an olefinically unsaturated monomer, the polyol is a polyester polyol and/or a polyether polyol, preferably being a polyether polyol. The stable dispersant obtained by means of the described preparation method has a multi-active site anchoring function, and is applied to the synthesis of copolymer polyols to obtain copolymer polyols having relatively uniform particle size.