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The microstructure of as-cast AZ91-3Ca magnesium alloy before and after annealing treatment and the corrosion behavior under different annealing conditions were studied by scanning electron microscopy (SEM), X-ray diffractometer (XRD), electrochemical test and weight loss method. The results show that the as cast AZ91-3Ca alloy is mainly composed of matrix a -Mg phase and a network eutectic phase along the grain boundaries. The eutectic phase contains a -Mg phase, a solid block β-Mg17Al12 phase and a Mg (Al) 2Ca phase. The microstructures of the alloys annealed at 300 ℃ and 400 ℃ have no obvious change. After annealing at 500 ℃ for 2.0 h, the solution of β-Mg17Al12 phase promotes the precipitation of Mg, Al, Zn and Ca new phase. After annealing at 500 ℃ for 16.0 h, the content of Al in the matrix is the most, and the distribution is uniform and the corrosion resistance is the best, in NaCl aqueous solution the corrosion potential was -1330 mV, the current density was 1.222 × 10-4 A / cm2, and the corrosion rate was 0.11 mg / cm2h.