A series of xM-Ce0.7Zr0.3O2 prepared by citric acid sol-gel method exhibited different catalytic activity for soot oxidation, which varies as the content and ionic radius of doping metals changed. 0.15 K-CZ had the best catalytic performance. The results of H-2-TPR, O-2-TPD and XPS demonstrate that the active oxygen species can be adjusted and controlled by doping alkali metals with different ionic radius into Ce0.7Zr0.3O2. Three kinds of active oxygen species are involved in the soot oxidation reaction sequence, and superoxide ionic is the key species.