A low-shrinkage polyester industrial yarn and a preparation method thereof are provided. The preparation method includes the following steps: subjecting the modified polyester to polycondensation, melting, measuring, extruding, cooling, oiling, stretching, heat setting and winding, wherein the content of the crown ether in the oil agent is 67.30-85.58 wt %. The material of the prepared low shrinkage polyester industrial yarn is a modified polyester, the molecular chain of the modified polyester includes a terephthalic acid segment, an ethylene glycol segment, and a branched diol segment, and the structural formula of the branched diol is as follows: Wherein R1 and R2 are each independently selected from a linear alkylene group having 1-3 carbon atoms, R3 is selected from an alkyl group having 1-5 carbon atoms, and R4 is selected from an alkyl group consisting of 2-5 carbon atoms.