Abstract: Assuming that the main effect of the voids is to reduce the area over which the stress acts in the early stage and the stress decreases to porosity-dependent value in the void coalescence stages, the stress relaxation equations in the spall process are given and a void coalescence-based spall model is proposed. The stress(or pressure)-dependent spall porosities given by Thomason, Tonks, et al and Cochran et al are respectively combined with conservation equations, equation of state and constitutive equations for the damaged aggregate to establish a series of close equations for all variables including damage. The void coalescence-based spall model is applied to numerical simulations of one-dimensional spall tests. The spall strength and the critical damage in the proposed spall model can be initially estimated and final determination of them should make the calcualted results of spall test under initial and boundary conditions consistent with the experimental velocity(stress) profile and the observed damage at spall plane. The calculated results based on the pressure-dependent spall porosities given by Tonk et al and Cochran et al are consistent but different from the calculated results based on Thomasons stress-dependent spall porosity.