The invention relates to an ultra-low shrinkage polyester industrial yarn and a preparation method thereof. The ultra-low shrinkage polyester industrial yarn is formed through spinning after polyester dissolving and washing and solid-phase tackifying; the drying shrinkage rate of the ultra-low shrinkage polyester industrial yarn under the test condition that the temperature is 177 DEG C*10 min*0.05 cN/dtex is 1.8+/-0.25%. The mixture of ethylene glycol magnesium and ethylene glycol antimony is taken as a polycondensation catalyst, the thermal degradation coefficient of the polycondensation catalyst is very small, oligomers in the polymerization process are reduced, the thermal degradation in the processing process is reduced, and impurities in the polyester are greatly reduced, meanwhile, the amount of a nucleating agent in the polyester is also reduced, the probability of homogeneous nucleation is increased on the basis of reducing heterogeneous nucleation, the content of the oligomers is further reduced through dissolving and washing, and the increase of grain size and crystallization completeness optimization in the ultra-low shrinkage polyester industrial yarn fiber are facilitated. The ultra-low shrinkage polyester industrial yarn has the characteristics of lower drying shrinkage rate, higher dimensional stability and the like, and is widely applied to the aspects of filter cloth, coated fabrics, hoses, conveyor belt weft and the like.