Activated semicoke (ASC) was modified with 3 wt % SiO2 and loaded with Fe-Co mixed oxides. This prepared catalyst exhibited excellent denitrification (deNO) activity, even when the flue gas contained 10 vol % water vapor, from 150 to 300 degrees C. To understand the water resistance mechanism, the catalysts were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman, X-ray, Fourier-transform infrared, and UV-vis spectroscopies, and H-2-temperature-programmed reduction, as well.as NO and/or Water adsorption testing. In addition, quantum mechanical calculations were performed. The results indicate that the SiO2 doped onto the semicoke surface repels both water and NO molecules. However, the repulsive force toward NO is smaller, resulting in the adsorption of NO on the surface, while the adsorption of water was reduced. The reduction in the qUaritity of adsorbed water improved the water tolerance of the catalysts significantly. The adsorption results demonstrate that, when the water adsorption was less than 0.345-0.442%, water vapor had no influence on the deNO activity of ASC supported catalysts.