A preparation method of a different shrinkage composite yarn, wherein a modified polyester is spun from a porous spinneret to produce a different shrinkage composite yarn, and an arrangement of spinneret holes in the porous spinneret is an elliptical arrangement; the elliptical arrangement means that the centers of the spinning holes of the spinneret are located on the concentric ellipses, the concentric ellipses are a series of ellipses, the long axes of all the ellipses are collinear, and the short axes of all the ellipses are collinear; said preparation method comprising: preparing the modified polyester POY yarn by metering, squeezing, cooling, oiling and winding the modified polyester, and the modified polyester FDY yarn is prepared by metering, extrusion, cooling, oiling, drawing, heat setting and winding the modified polyester chips; combining the modified polyester POY yarn and the modified polyester FDY yarn as a network structure to form the different shrinkage composite yarn; wherein the preparation steps of the modified polyester are as follows: preparation of terephthalic acid glycol ester: mixing terephthalic acid and branched diol to a first slurry, and carrying out a first esterification reaction under the catalytic action of concentrated sulfuric acid to obtain a terephthalic acid glycol ester; the branched diol comprising a diol having a branched chain which is a non-terminal carbon in the diol segment and a branched chain having a linear carbon chain of 5 to 10 carbon atoms; preparation of ethylene terephthalate: mixing the terephthalic acid and the ethylene glycol to a second slurry, carrying out the esterification reaction to obtain ethylene terephthalate; preparation of modified polyester: after the second esterification reaction in the step is completed, adding the terephthalic acid glycol ester prepared in the step, stirring and mixing, performing a polycondensation reaction under the action of the catalyst and the stabilizer, and under the condition of a negative pressure, first in a low vacuum stage and then in a high vacuum stage to obtain a modified polyester. preparing the modified polyester POY yarn by metering, squeezing, cooling, oiling and winding the modified polyester, and the modified polyester FDY yarn is prepared by metering, extrusion, cooling, oiling, drawing, heat setting and winding the modified polyester chips; combining the modified polyester POY yarn and the modified polyester FDY yarn as a network structure to form the different shrinkage composite yarn; wherein the preparation steps of the modified polyester are as follows: preparation of terephthalic acid glycol ester: mixing terephthalic acid and branched diol to a first slurry, and carrying out a first esterification reaction under the catalytic action of concentrated sulfuric acid to obtain a terephthalic acid glycol ester; the branched diol comprising a diol having a branched chain which is a non-terminal carbon in the diol segment and a branched chain having a linear carbon chain of 5 to 10 carbon atoms; preparation of ethylene terephthalate: mixing the terephthalic acid and the ethylene glycol to a second slurry, carrying out the esterification reaction to obtain ethylene terephthalate; preparation of modified polyester: after the second esterification reaction in the step is completed, adding the terephthalic acid glycol ester prepared in the step, stirring and mixing, performing a polycondensation reaction under the action of the catalyst and the stabilizer, and under the condition of a negative pressure, first in a low vacuum stage and then in a high vacuum stage to obtain a modified polyester. preparation of terephthalic acid glycol ester: mixing terephthalic acid and branched diol to a first slurry, and carrying out a first esterification reaction under the catalytic action of concentrated sulfuric acid to obtain a terephthalic acid glycol ester; the branched diol comprising a diol having a branched chain which is a non-terminal carbon in the diol segment and a branched chain having a linear carbon chain of 5 to 10 carbon atoms; preparation of ethylene terephthalate: mixing the terephthalic acid and the ethylene glycol to a second slurry, carrying out the esterification reaction to obtain ethylene terephthalate; preparation of modified polyester: after the second esterification reaction in the step is completed, adding the terephthalic acid glycol ester prepared in the step, stirring and mixing, performing a polycondensation reaction under the action of the catalyst and the stabilizer, and under the condition of a negative pressure, first in a low vacuum stage and then in a high vacuum stage to obtain a modified polyester.