Formaldehyde is an important air pollutant and its removal is essential to protect human health and meet environmental regulations.Ag-based catalyst has a considerable potential for HCHO oxidation in low temperature range.The valence state of Ag is one of the key roles in formaldehyde catalytic oxidation.However,its effect on activity is still ambiguous.Non-thermal plasma and conventional calcination were employed to regulate Ag valence state in this study.Three Ag-Co/CeO2 catalysts with totally different distribution of Ag species were obtained.A special mixed Ag valence state,～50％ Agδ+ with a few Ag0 and Ag+,was achieved by plasma activation.It had the merits of both good activity and stability.A close relationship between Ag valence state and the activity for HCHO oxidation was established.The activity of different Ag species follows the order:Agδ+ + Ag0 + Ag+ ＞ Agδ+ ＞ Ag0 ＞ Ag+.