The selective oxidation of dimethyl ether (DME) to methyl formate (MF) was conducted in a fixed-bed reactor over the MoO₃–SnO₂ catalysts with different Mo/Sn ratios. The MF selectivity reached 94.1% and the DME conversion was 33.9% without the formation of CO_x over the MoSn catalyst at 433 K. The catalysts were deeply characterized by NH₃-TPD, CO₂-TPD, BET, XPS and H₂-TPR. The characterization results showed that different compositions of catalysts obviously affected the surface properties of the catalysts, but the valence of the metal hardly changed with the Mo/Sn ratios. Raman spectroscopy, XRD and XAFS were further used to characterize the structure of the catalysts. The results indicated that the catalyst composition exerted a significant influence on the structure of MoO₃. The formation of oligomeric MoO₃ and the appropriate coordination numbers of Mo–O at 1.94 Å are the main reasons for the distinct high catalytic activity of the MoSn catalyst.