ITO surface photochemical reaction induced by soft X-ray (Mg Kα=125360eV) irr adiation is investigated in-situ by XPS technique. The result shows that the con tent of In and Sn in the irradiated area goes up with the increasing irradiation time, while that of O falls. The changes of In3d, Sn3d XPS spectra and the Auge r Parameter of In, Sn suggest that there is chemical reaction due to X-ray irrad iation. It can be concluded that more photo-dissociation happens to In element a ccording to the obvious fact that there exists sub-oxidized state of In after ex posure to X-ray. The photochemical reaction mechanism induced by soft X-ray irra diation is discussed.