The viscoplastic collapse mechanism of hot-spot formation in pressed porous TNT charges was studied.Amodel was proposed and numerically calculated based on the viscoplastic motion and induced heat of the media around cavities due to the meIting effect.The resuIts showed that in moIten region near the boundary of cavities with different dimensions the temperature was kept near the meIting point, and the existence of the molten region ensured a heat layer with a certain depth under the shock pressure with amplitude of about 3 GPa.Due to the enlargement of molten region during the collapse of cavities,the yield stress tended to zero and viscoplativity was very small in the molten region,these factors may result in a high velocity of cavity boundary collapse,therefore on the close stage of cavity the fluid-dynamics mechanism of ignition must be considered for these samples from large granular,while the melt remarkably decreased the sensitivity of ignition for these samples from fine granular.