The invention relates to a low-shrinkage high-strength polyester industrial yarn and a preparation method thereof. The preparation method comprises: carrying out solid phase condensation polymerization tackifying on a modified polyester, melting, metering, extruding, cooling, oiling, drawing, carrying out heat setting, and winding, wherein the unchanged longitudinal height is maintained during thecooling, the cross section area of the slow cooling chamber is increased while the plate surface temperature of the spinning plate is maintained by using the thermal insulation method, and the oil agent for oiling contains 67.30-85.58 wt% of crown ether. According to the present invention, the preparation method is simple; the material of the prepared low-shrinkage high-strength polyester industrial yarn is the modified polyester with the molecular chain including a terephthalic acid chain segment, an ethylene glycol chain segment and a diol chain segment having the branched chain, the fracture strength is more than or equal to 7.3 cN/dtex, and the dry heat shrinkage rates at the conditions comprising 177 DEG C, 10 min and 0.05 cN/dtex and the conditions comprising 190 DEG C, 15 min and 0.01 cN/dtex respectively are 1.8+/-0.5% and 3.5+/-0.5%.