The purpose of the present work is to investigate the deformation failure behavior of tungsten alloy long rob penetrating into armor target by metallographic analysis and numerical simulation. Microstructure evolution of the residual projectile was studied using SEM and OM. The results show that mushroom-like-head of penetrator is formed during penetration due to severe plastic deformation, and W grains in the mushroom are elongated heavily. An adiabatic shear band is observed perpendicular to the penetration direction in the front mushroom about 1 mm, in which W grains seem dissolving due to local temperature rise, and micro-cracks are developed both in W grains and W-W interface. Numerical simulation shows that the mushroom forms and falls off continuously during penetrating, so the long rob becomes shorter for the erosion. Severe plastic deformation in the mushroom is similar to plastic flow, and the deformation localization occurs mainly upon the both sides or the front of the mushroom.