The invention discloses polyester, which is prepared by alcoholyzing waste polyester materials, used in powdery coatings and a preparation method thereof. The preparation method includes the steps of: 1) regulating the size and distribution of molecular weight of a diol compound, which is prepared through alcoholysis, through selection on content of a catalyst and process temperature with the waste polyester materials as a raw material and 1,3-diethyl imidazole borofluorate as the catalyst; 2) when the size and distribution of the molecular weight are regulated in a proper range, adding p-phthalic acid, m-phthalic acid and the catalyst according to a formula to complete stepwise polymerization; and 3) performing polycondensation of the pure polyester through vacuumizing, thereby finally producing the pure polyester which is low in melt viscosity, high in glass transition temperature and narrow in molecular weight distribution and is used for the powdery coatings. A powdery coating produced with the pure polyester is better than a coating in the prior art in flatting performance and glossiness.