Abstract: Based on the basic phase diagram,studies concerning determination of the C content of carbon steel peritectic point （C_J） via plotting and calculating methods. Also discussed in the paper is the effect on the C_J value by contents of Si and Mn in the steel. Relevant operation data have proven that the closer to the C_J value the C content in the steel is, the more possible thermal.cracking of the CC round will be developed. An approach to preventing the C content from getting close to the C_J value is set up.