AZ91magnesium alloys modified by erbium (Er) and cerium (Ce) (AZErCe)were fabricated by casting into metal mold. The effect of Er and Ce on the microstructure and stress corrosion cracking (SCC) behavior of AZ91 in Cl- containing moist atmosphere was investigated by slow strain rate tensile (SSRT) tests, potentiodynamic polarization analysis, X-ray photoelectron spectroscope (XPS), scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that the β-Mg17Al12 phase was transformed from discontinuous network into small island distribution and dispersed homogeneously after the addition of Er and Ce, and the formation of Al3Er and Al11Ce3 intermetallic compounds reduced the volume fraction of β-Mg17Al12 phase.The addition of Er and Ce greatly improved the SCC resistance, because the local corrosion resistance of AZErCe was improved by alleviating the cathodic effect of β-Mg17Al12 phase and improving the compactness of the surface films. As a result, the processes of initiation and expansion of cracks were suppressed and the influence of hydrogen embrittlement on SCC resistance was weakened. The fractography of SCC was changed from brittle cleavage fracture into quasi-cleavage fracture, and the local corrosion at the edge of fracture surfaces was greatly alleviated.