The invention relates to a polyester-polypentamethylene diamine diacid blocked copolymer and a preparation method and fiber thereof. The preparation method comprises the following steps: firstly, preparing ethyleneglycol p-phthalate, performing condensation polymerization reaction to the ethyleneglycol p-phthalate to obtain an ethyleneglycol p-phthalate oligomer with the polymerization degree of 5-20, then performing condensation polymerization reaction to pentametylenediamine diacid salt water solution, to obtain pentametylenediamine diacid oligomer with the polymerization degree of 2-10, and finally, mixing the two oligomers and performing condensation polymerization reaction, to obtain polyester-polypentamethylene diamine diacid blocked copolymer, wherein the mass ratio of ethyleneglycol p-phthalate block to polypentamethylene diamine block in the copolymer is (90-10):(10-90). Fibers can be prepared by pre-crystalizing, drying, spinning and drafting the polyester-polypentamethylene diamine diacid blocked copolymer, and the elasticity and hygroscopic properties of the fiber are excellent, the elastic recover rate when the fixed elongation rate is 10%, and the moisture regain is 0.6-5.0%.