Abstract: Deactivation of composite catalyst for onestep dimethyl ether (DME) synthesis in slurry reactor was studied under reaction conditions of 260℃ and 5.0MPa. It was found that instability of Cubased methanol synthesis catalyst led to rapid deactivation of the composite catalyst. Deactivation rate of the Cubased catalyst in slurry reactor was compared with that in fixedbed reactor. The results indicated that harmfulness of water, which is formed in the synthesis of DME, caused the Cubased catalyst to deactivate at a high rate in slurry reactor. Techniques of TPR, XRD, and SEMEDS were used to characterize the reduction behaviors, crystal structures, and surface properties of the catalyst. The results showed that carbon deposition and grain growth of Cu were important reasons for the rapid deactivation of the Cubased catalyst, and no obvious metal loss of Cu was found.