The dependence of shrinkage porosities on microstructure characteristics of Mg−12Al alloy was investigated. The distribution, morphology, size, and number density of shrinkage porosities were analyzed under different cooling rates. The relationship between shrinkage porosities and microstructure characteristics was discussed in terms of temperature conditions, feeding channel characteristics, and feeding capacity. Further, the feeding behavior of the residual liquid phase in the solid skeleton was quantified by introducing permeability. Results show a strong correlation between the solid microstructure skeleton and shrinkage porosity characteristics. An increase in permeability corresponds to a declining number density of shrinkage porosities. This study aims to provide a more complete understanding how to reduce shrinkage porosities by controlling microstructure characteristics.