The properties of titanium carbonitride (Ti(CxN1-x)) inclusions precipitated during solidification of tire cord steel, as well as thermodynamic conditions of decomposition and solid solution of titanium carbonitride (Ti(CxN1-x)) inclusions in the billet heating process are studied by thermodynamics. The solid solution phenomenon of titanium carbonitride inclusions in the high temperature heating process is studied with lab experiment. The results show 1) The higher the carbon content in tire cord steel, the higher the value of x in titanium carbonitride inclusions; 2) The higher the carbon content in tire cord steel, the earlier the titanium carbonitride inclusions precipitated during solidification process, the lower solidfication front temperature of precipitation of titanium carbonitride inclusions. 3) When heated to 1 087 ℃, the titanium carbonitride inclusions of 82A billet possess the thermodynamic conditions of decomposition and solid solution; 4) When heated to 1 150 ℃ and 1 250 ℃, the content of titanium carbonitride inclusions with size more than 5 μm are decreased by 55% and 70.3% respectively; However the small inclusions with size less than 2 μm continue to decompose, and the content of inclusions with size more than 5 μm increases when the sample is heating at 1 250 ℃ for two hours then furnace cooling to 1 000 ℃.