Based on the current situation of studying the thermodynamic property of Fe-C-Cr melt using the carbon saturated solubility, an experimental data treatment method of the carbon saturated solubility was put forward. With this method a linear relationship ex-pression of the carbon saturated solubility in Fe-C-Cr melt was obtained, which intercept is dependent on temperature and independent of third component[Cr], but which slope is dependent on third component[Cr] and independent of temperature. Through this expression activity interaction coefficients at different temperatures were calculated and the relationship between activity interaction coefficients and temperature is also obtained.