Abstract: Due to high Cr and C contents,there are a mass of network eutectic carbides precipitating at grain boundaries in the as-cast Cr12Mo1V1 cold working die steel. After forging,most of them are long strip-shaped and have a zonal distribution,leading to a decrease in thermoplasticity,easy formation of cracks,and thus the limit of application. The effects of Mg addition on the carbides and thermoplasticity of the Cr12Mo1V1 steel were investigated in the present study. It is found that with increasing Mg content in the steel,network eutectic carbides in the as-cast steel are interrupted and refined,and can be shattered more easily during the later forging procedure. Meanwhile,the average size of carbides in the forged steel decreases with increasing Mg content,and these carbides present a uniform diffuse distribution. The improvement in size and distribution of carbides enhances the thermoplasticity of the steel.