At the present stage, the development of spallation damage research is restricted because of the lack of real-time experimental means to capture the variation of void growth and its distribution characteristics and the effective mathematical description method to describe the variation of pore distribution characteristics. Under strong impact loading, the spallation damage evolution of ductile materials includes physical processes such as nucleation, growth, coalescence and finally 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 between voids is mainly by 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 the nucleation of voids is described in the form of cosine trigonometric function, and the reduction of void number due to the coalescence of voids is described in the form of sine trigonometric function. The phenomenological physical description method of the whole process of the variation of the number density of voids is given, and then the evolution equation of spallation damage coupled with the variation law of the number density of voids is constructed. The new model not only fully reflects the physical process of nucleation, growth and coalescence of voids, but also shows the changing law of the distribution characteristics of voids in damage evolution. The calculated results can give the damage state and the distribution of the number density of voids in the material, which provides an effective support for the analysis of 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.