Abstract: A polyester polyol is synthesized through an esterification reaction with a sebacic acid by-product fatty acid as a raw material; wherein the sebacic acid by-product fatty acid is refined from a by-product produced during preparing a sebacic acid from a castor oil; the sebacic acid by-product fatty acid includes, in weight percentage: a palmitic acid 15-25%, a stearic acid 10-16%, an oleic acid 45-57%, and a linoleic acid 12-28%. A method for preparing the polyester polyol is provided, as well as a polyurethane controlled-release fertilizer envelope and a polyurethane controlled-release fertilizer. The sebacic acid by-product fatty acid is used as a raw material to synthesize the polyester polyol because of a low price. The prepared fertilizer has excellent envelope and controlled-release performance, product structure performance is stable, cost performance is high, and degradation performance is sufficient after being applied to soil.