A method of preparation of a porous ultra-soft ultra-fine denier polyester fiber, wherein the modified polyester is spun from a porous spinneret to produce a porous ultra-soft ultra-fine denier polyester fiber, and the arrangement of the spinneret holes in the porous spinneret is an elliptical arrangement; the elliptical arrangement meaning that the centers of the spinning holes of the spinneret are located on the concentric ellipse, the concentric ellipses being a series of ellipses, the long axis collinear of all the ellipses are collinear, and the short axes are collinear; the preparation method of the porous ultra-soft ultra-fine denier polyester fiber comprising: granulating the modified polyester to obtain modified polyester chips, then obtaining porous ultra-soft ultra-fine denier polyester fiber by metering, extruding, cooling, oiling and high-speed winding the modified polyester chips; wherein, a method for preparing the modified polyester chips comprises: preparation of terephthalic acid glycol ester: mixing terephthalic acid and branched hexanediol to first slurry, and carrying out a first esterification reaction under a catalytic action of concentrated sulfuric acid to obtain a terephthalic acid glycol ester. The branched hexanediol refers to a diol having a branched chain which is a non-terminal carbon in the ethylene glycol 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 a second esterification reaction to obtain the 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 a condition of a negative pressure, first in a low vacuum stage and then in a high vacuum stage to obtain a modified polyester. granulating the modified polyester to obtain modified polyester chips, then obtaining porous ultra-soft ultra-fine denier polyester fiber by metering, extruding, cooling, oiling and high-speed winding the modified polyester chips; wherein, a method for preparing the modified polyester chips comprises: preparation of terephthalic acid glycol ester: mixing terephthalic acid and branched hexanediol to first slurry, and carrying out a first esterification reaction under a catalytic action of concentrated sulfuric acid to obtain a terephthalic acid glycol ester. The branched hexanediol refers to a diol having a branched chain which is a non-terminal carbon in the ethylene glycol 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 a second esterification reaction to obtain the 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 a 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 hexanediol to first slurry, and carrying out a first esterification reaction under a catalytic action of concentrated sulfuric acid to obtain a terephthalic acid glycol ester. The branched hexanediol refers to a diol having a branched chain which is a non-terminal carbon in the ethylene glycol 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 a second esterification reaction to obtain the 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 a condition of a negative pressure, first in a low vacuum stage and then in a high vacuum stage to obtain a modified polyester.