Considering the brittleness of tungsten alloys as pre-fabricated fragment ammunition after armour-piercing, tungsten alloy spheres embedded in cylinders were employed; the microstructure and fracture mechanism of 97.5W-Ni-Fe alloy were investigated under explosive loading before and after armour-piercing. The results show that high strain rate explosive loading induces a large amount of deformation twins in tungsten particles. The twining deformation after armour-piercing induces lots of micro-cracks and these micro-cracks extend to connect with the W-W interface fracture, becoming macro-cracks due to the tensile stress. As a result, the tungsten alloy spheres are fractured to efficient fragments and the second damage ability is enhanced.