The influence of rare earth element (RE) Y on the microstructure and corrosion behavior of extruded AZ91 Mg alloy was surveyed via morphology characterization and corrosion performance measurements. The results indicate the corrosion resistance of the transversal section of AZ91 Mg alloy containing Y was improved compared with AZ91 Mg alloy without Y. The corrosion resistance of the longitudinal section of AZ91 Mg alloy with Y was lower than that of AZ91 Mg alloy without Y. The change of corrosion resistance can be attributed to the dispersion and volume fraction of the second phase, the effect of cathodic reduction rate, and the refined second phase.