Based on the research works of home and abroad, a casting flux was developed according to the bloom continuous casting process of medium and low carbon steel at PZH Steel. The coordination of glasslike - crystal property, capacity of absorbing inclusions and carbon mixing technology of casting flux were researched as key contents in laboratory, a reasonable slag system of casting flux for medium and low carbon steel bloom continuous casting were determined. Industrial - scale experiments showed that the developed casting flux could meet the technological requirements of the bloom continuous casting.