Abstract: The effect of cooling intensity on the carbide segregation and solidification microstructure of an H13 as-cast ingot with Mg addition was studied during the electroslag remelting process. The solidification microstructure and the carbide characteristics of the steel were analyzed by optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis. The results show that the solidification microstructure of the ingot consists of martensite, retained austenite and primary carbides. Carbides in the electroslag remelting ingot are V₈C₇, MC, M₂₃C₆ and M₆C. With the increasing of cooling intensity, the size of carbides in the edge of the electroslag remehing ingot decreases, the distribution of these carbides becomes more homogeneous, but the types of carbides remain unchanged. Increasing the cooling intensity improves the modification of inclusions by Mg. The formed MgO. Al₂O₃ arising from Mg treatment provides preferred nucleation sites for TiN. These complex inclusions promote the heterogeneous nucleation of primary carbides, consequently refining primary carbides.