The difficulty of maintaining steady reaction is generated when water presents on supported Wacker-type catalyst in CO oxidation. Water can be adsorbed and condensed on catalyst surface, which facilitated Cu species transporting from the surface into the inner part of support. This weakened the contact between Pd and Cu species and caused their existence in low valence. The temperature difference between the co-reduction peak of Pd and Cu species and the individual Cu species in H-2-TPR profile was found to have a linear relationship with the extent of deactivation. A mathematic model was developed and the effects of flow rate, CO concentration, temperature and relative humidity were also analyzed. The mathematic model well described the performance of CO oxidation at high relative humidity. Combined with the characteristic and modeling results, it was known that irreversible deactivation played a key role at lower temperature/higher relative humidity; while reversible deactivation was responsible for the instability at higher temperature/lower relative humidity. (C) 2014 Elsevier B.V. All rights reserved.