The microstructures and discharge products of Mg-6%Al-3%Zn and Mg-6%Al-3%Zn-0.5%In alloys are investigated by morphology observation, phase characterization, and composition analysis. Hydrogen collection, weight-loss measurement, and electrochemical tests are conducted to discuss their corrosion and discharge behaviors. An indium aroused dissolution–reprecipitation mechanism for Mg activation is proposed. Results suggest that the dissolution–reprecipitation mechanism can suitably interpret the indium-activation effect. Indium addition into Mg promotes Mg dissolution through synergistic effects, which involve increasing the second-phase amount, generating less-protective products, promoting products self-peeling, and dissolution–reprecipitation of indium. The homogenized Mg-Al-Zn-In alloy performs desirable corrosion resistance and discharge capability and thus is a promising candidate for applications as anode materials.