A method for preparing high shrinkage PTT copolyester, comprising: adding starting raw materials into a reaction kettle and adding an esterification catalyst to carry out esterification reaction at 220° C.˜260° C. under 0.05˜0.30 MPa and completing the reaction when there being no water produced: adding polymerization catalyst into the reaction system and carrying out the polymerization reaction at 240° C.˜280°C. to get polytrimethylene terephthalate. The starting raw materials consists of 1,3-propylene glycol, terephtalic acid, a third monomer, a fourth monomer and a fifth monomer. The third monomer is selected from the group consisting of non-para aromatic dicarboxylic acid, 1,4-cyclohexane diformic acid and glutaric acid; The fourth monomer is selected from the group consisting of C3-C10 branched aliphatic dihydric alcohol, 1,4-cyclohexanedimethanol, diethylene glycol and triethyleneglycol; The fifth monomer is aliphatic polyalcohols which has carbon number between 3 and 10 and hydroxyl number per molecule is more than or equal to 3, or trimellitic anhydride, amount of the 1,3-propylene glycol, terephthalic acid, third monomer, fourth monomer and fifth monomer are such used in a molar ratio of carboxyl to hydroxyl is 1:1.16˜-1.85, wherein the amount of the third monomer used is 5%-10.5% of the total weight of the starting raw materials, the amount of the fourth monomer used is 0.05%-1.0% of the total weight of the starting raw materials, and the amount of the fifth monomer used is 50 ppm-600 ppm, based on the total weight of the starting raw materials. Polytrimethylene terephthalate prepared by the invention shows good spinning behaviour, of which the crystallization speed is noticeably lower than that of conventional polytrimethylene terephthalate, thus, fibers made of them has high boiling water shrinkage rate and ideal shrink property.