At the current stage, the development of spallation damage research is limited because of the lack of real-time experimental methods to capture the variation of void growth and its distribution characteristics, as well as effective mathematical description method to describe the variation of pore distribution characteristics. Under strong impact loading, the evolution of spallation damage in ductile materials includes physical processes such as nucleation, growth, coalescence, and final fracture/fragmentation of materials. The growth of voids basically maintains the expansion of spherical holes. The damage evolution process can be divided into two stages: nucleation and growth of voids, and coalescence and growth of voids. The coalescence of voids occurs mainly through direct impingement. Based on the analysis of the variation law of the number of voids in the spallation damage evolution simulated by molecular dynamics, the probability distribution of void nucleation is described using a cosine trigonometric function, and the reduction of void number due to the void coalescence is described using a sine trigonometric function. A phenomenological physical description method for the whole process of the void number density variation is given, and then an evolution equation for spallation damage coupled with the variation law of void number density is constructed. The new model not only fully reflects the physical processes of nucleation, growth and coalescence of voids, but also shows the changing law of void distribution characteristics during damage evolution. The calculation results can determine the damage state and the distribution of void number density in the material, which provides effective support for the analyzing the recompression and fragmentation of materials after spall damage. At the same time, it also promotes the development of spallation damage research. The applicability of the new model is validated by the statistical results of microscopic molecular dynamics computation and related experimental results.