Disclosed is a high strength polyester fiber for a seat belt, and in particular, a polyester fiber for a seat belt, which has intrinsic viscosity of 0.8 to 1.5 dl/g, tensile strength of 8.8 g/d or more, and total fineness of 400 to 1800 denier. A method of preparing the fiber is disclosed. The polyester fiber includes filaments having high strength, low modulus, and high elongation to significantly lower shrinkage, while securing excellent mechanical properties, it is possible to manufacture a seat belt having excellent impact absorption and significantly improved abrasion resistance and heat resistant strength retention, even with a woven density of 260 yarns/inch or less.