The invention discloses a preparation method of polyester resin used in outdoor weatherable powder paint. The method comprises the steps that: (1) washed, impurity-removed, and crushed PET raw material is subjected to a trans-esterification reaction with dihydric alcohol with a boiling point higher than that of ethylene glycol under the existence of a trans-esterification catalyst; after the trans-esterification reaction is finished, polyol, polyacid/anhydride, and a stabilizer are added; an esterification reaction is carried out under the effect of an esterification catalyst; isophthalic acid and a curing accelerator triphenylphosphine are added, and a stirring reaction is carried out, such that the polyester resin used in outdoor weatherable powder paint is obtained. The polyester resin has the characteristics of outdoor weatherability, baking resistance, salt spray resistance, and the like of a powder paint synthesized and produced by using weatherable polyester resin, and has good gloss, impact resistance, and excellent leveling performance. The pollutions to the environment caused by polyester waste material and esterification water produced in purified terephthalic acid synthesis are reduced. Therefore, the technical scheme provided by the invention is environment-friendly.