This paper provides a calculation method for carbon activity in CO-H2-H2O atmosphere. The thermodynamic parameters (αC>)gas (carbon activity in environment) of different compositions at any temperature can be obtained by ΔGoT. A theoretical analysis has been conducted into the thermodynamic role of iron and the dependence of possible metal-dusting occurrence on temperature, gas composition and total pressure. In CO-H2-H2O gas mixtures, decreasing the molar fraction of H2O and increasing total pressure expands the temperature region for metal dusting. In CO-H2-H2O gas mixture of different compositions at any temperature and pressure for Fe, depending on relative values of (αC)gas, (αC)Fe3C/Fe and αC=1, three zones were found to exist.