The invention relates to a structural transformation control method in a continuous cooling process of an ultra-low-carbon wire rod of a square billet, belonging to the technical field of steel rolling. Aiming at the structural transformation in the continuous cooling process after thermal simulation transformation is carried out on a hot-rolling steel wire rod for a copper-clad steel wire, a continuous cooling structural transformation curve of the hot-rolling steel wire rod for the copper-clad steel wire is analyzed through structural transformation characteristics in the continuous coolingprocess under different cooling rates after the hot transformation. The invention solves the problems of cooling process and structural control after the hot-rolling steel wire rod for the copper-clad steel wire rod is rolled, and provides data support on crystallite dimension control and tertiary cementite precipitation number for rolling and producing the ultra-low-carbon wire rod of the squarebillet.