A type of fully dull polyester drawn yarns and a preparing method thereof are disclosed. The preparing method is to melt spinning a modified polyester with the fully drawn yarn (FDY) technique, and the modified polyester is a product of an esterification and successive polycondensation reactions of evenly mixed terephthalic acid, ethylene glycol, 2,5,6,6-tetramethyl-2,5-heptanediol, a fluorinated dicarboxylic acid, a matting agent, a calcined multiphase solid acid base powder and a doped Bi2O3 powder. The obtained fiber has an intrinsic viscosity drop of 18-26% when stored at 25° C. and R.H. 65% for 60 months. The method of improving the degradation performance of polyester fiber through the incorporation of 2,5,6,6-tetramethyl-2,5-heptanediol, the fluorinated dicarboxylic acid, the doped Bi2O3 powder and the calcined multiphase solid acid base powder is easy to operate.
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