The present invention relates to a high strength polyester fiber for a seat belt, and in particular, to a polyester fiber for a seat belt having 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, and a method of preparing the same. As the polyester fiber of the present invention includes filaments having high strength, low modulus, and high elongation to significantly lower shrinkage, while securing excellent mechanical properties, a seat belt having excellent impact absorption and significantly improved abrasion resistance and heat resistant strength retention may be manufactured even with a woven density of 260 yarns/inch or less.