A facile one-step co-precipitation method has been applied for the synthesis of a Pt decorated octahedral Fe3O4 catalyst. The simple addition of a Pt(4+) and Fe(2+) mixture into a KOH solution leads to the simultaneous formation of an octahedral Fe3O4 and in situ reduction of Pt(4+). HAADF-STEM analysis demonstrates the good dispersion of the Pt species on the Fe3O4 (111) plane, and the resulting material exhibits excellent catalytic activity for CO oxidation under moisture conditions. The inevitably existing moisture contributes to the formation of reaction intermediate [COOH] and hence promotes the catalytic activity, which has been proved through in situ DRIFTS analysis.