A high uniformity coloured polyester industrial yarn and preparation method. The raw material of the high-uniformity coloured polyester industrial yard is the modified polyester composed of terephthalic acid segment, an ethylene glycol segment and a branched diol. The high-uniformity coloured polyester industrial yarn is prepared by esterification reaction of terephthalic acid with a branched diol using concentrated sulfuric acid as catalyst to obtain terephthalic acid glycol ester. Terephthalic acid and ethylene glycol are formulated to carry out esterification reaction to obtain ethylene terephthalate. Finally, stir and mix the two materials and carry out the polycondensation reaction in the low vacuum stage and the high vacuum stage under the action of the catalyst and the stabilizer to obtain modified polyester. Measure the modified polyester and add black masterbatch. After porous spinneret extrusion, cooling, oiling, drawing, heat setting and winding the modified polyester, high uniformity coloured polyester industry yarn is obtained.