A rubber-reinforcing polyester fiber composed of a polyester prepared by using 30 to 150 ppm as antimony of an antimony compound and 5 to 60 ppm as germanium of a germanium compound as the polymerization catalyst. The fiber is characterized in that the terminal carboxyl group content is not larger than 25 eq/ton, diethylene glycol content (DEG) is not larger than 1.3% by weight, the intrinsic viscosity (IV) is at least 0.85, the sum (S) of the intermediate elongation and the dry heat shrinkage is lower than 8%, the product between strength and elongation (T~E) is at least (2S + 5), and the terminal modulus (TM) is not higher than 40 g/d. The polyester fiber is used as a rubber reinforcement.