The invention relates to porous PBT fiber POY yarn and a preparation method thereof. Modified PBT polyester is spun through a porous spinneret plate, so that the porous PBT fiber POY yarn is obtained; the arrangement mode of spinneret orifices on the porous spinneret plate adopts elliptic arrangement, that is, the centers of the spinneret orifices are positioned in concentric ellipses, the concentric ellipses adopt series ellipses, the long axes of all the ellipses are collinear, and the short axes of all the ellipses are also collinear. The porous PBT fiber POY yarn is formed by performing metering, extruding, cooling, oiling and coiling on the modified PBT polyester; the preparation method of the modified PBT polyester comprises the following steps: terephthalic acid and 1, 4-butanediol are reacted, so that terephthalic acid 1, 4-butylene terephthalate is obtained; terephthalic acid is added and reacted with dihydric alcohol containing branches, so that terephthalic acid dihydric alcohol ester is obtained; reaction is continued, so that the modified PBT polyester is obtained. The fiber prepared through the method is excellent in properties, the linear density variation rate is smaller than or equal to 0.2 percent, the breaking strength CV value is smaller than or equal to 2.5 percent, and the breaking elongation CV value is smaller than or equal to 5.0 percent.