Abstract: Based on the currently-used melting and rolling processes,two X65MS anti-acid linepipe steels with different C and Mn contents are designed so as to analyze the effects by C and Mn contents on HIC resistance of the X65MS anti-acid linepipe steel. The design principle of the chemical composition of different steel types is described,and the cast billet macrostructure,rolled plate microstructure and HIC resistance of the comparative test steels are inspected and measured. The analysis result demonstrates that with well-controlled extra-low P and S contents,and the excellent cleanness level,the low C and Mn content design is more beneficial to proper control of the central segregation of the cast billet,and gets the microstructure of the rolled steel plate more unitary and more stable,and free of banded structure,and have much better HIC resistance,being worth suitable for developing the high grade anti-acid steel linepipe.