The effects of Ca on the microstructures and the corrosion resistance of as-cast AZ91 alloys with contents of 0.5 wt%, 1.0 wt% and 1.5 wt% Ca were investigated. Constant immersion technique, potentiodynamic polarization and salt spray test were employed to determine the corrosion resistance of AZ91 alloys. Morphological characterization by optical microscope (OM), microanalysis by scanning electron microscopy (SEM)/energy dispersed spectroscopy (EDS) and phase identification by X-ray diffractometry (XRD) indicate that the presence of 0.5 wt% Ca does not form any new intermetallic phase but restrains the discontinuous precipitation of the β-Mg17Al12 phase through dissolution in both the second phase and magnesium matrix. The AZ91 alloy with the addition of 1.0 wt% Ca exhibits the optimal corrosion resistance. Al4Ca phase in bone-like morphology appears in the alloys with a larger quantity of Ca (1.0 wt% and 1.5 wt%), in line with a marked size reduction of the β-Mg17Al12 phase. The β-Mg17Al12 phase is distributed uniformly among the matrix of the AZ91-1.0Ca alloy. The variation of the corrosion resistance of the as-cast AZ91 alloys with the addition of Ca is attributed to the change of the microstructure.