Casting magnesium alloys are heterogeneous materials containing numerous voids. These voids markedly affect the mechanical behaviour of the materials. In this paper, a void-growth equation is obtained based on the analysis of a spherical void-cell model, and a void-nucleation equation is presented which is related to the increment of intrinsic-time measure. The evolution equation of the voids is obtained by combining the growth equation with the nucleation equation. The obtained void-evolution equation is incorporated into a nonclassical elastoplastic constitutive equation through introducing a softening function, thus obtaining a constitutive equation that involves a void evolution. A corresponding finite element procedure is developed and applied to the description of the rule of the void evolution and the mechanical behaviour of casting magnesium alloy ML308. Computed results are shown to be in satisfactory agreement with experimental data.