The invention relates to a preparation method for a modified PTT polyester chip, the preparation method comprises the esterification reaction step and the polymerization reaction step, and inorganic nanoparticles which account for 0.03-0.5 percent of the total weight of the chip are particularly added in a polymerized monomer after the completion of the esterification reaction and before the polymerization reaction. The inorganic nano-particles are selected from one or the mixture of more of nano-calcium carbonate, nano-silica and nano-magnesium carbonate. A small number of inorganic nanoparticles are added in the polymerization process to be used as a nucleating agent, the prepared PTT polyester chip has good quality, the melting point is the same with the conventional PTT polyester chip, and the color phase is good; the modified PTT polyester chip further represents better spinnability than the ordinary big bright PTT polyester chip, fibers obtained by spinning have higher strength and can realize the even dyeing; in addition, the method has high finished product rate of the PTT chip, simplicity, easy operation and low production cost.