The invention belongs to the field of steel-making continuous casting production of micro-alloy high-strength steel, and discloses a method for improving the high temperature plasticity of steel basedon a synergistic effect of magnesium treatment and two-times cooling. By the synergistic effect of a magnesium treatment process and a novel two-times cooling control process, components, the size and the distribution state of second-phase particles in a micro-alloy high-strength steel casting blank can be effectively controlled, the thickness of a proeutectoid ferrite film distributed along an original austenite grain boundary is controlled, the percentage reduction of area of the casting blank is increased significantly through detection with Gleeble3800 and is higher than 50%. Thus the effects that method can significantly improve the high temperature plasticity of the micro-alloy high-strength steel and the surface crack defects of the casting blank can be overcome effectively are verified.