In order to compare the similarities and differences among the self-sharpening behavior as well as the corresponding ballistic performance of tungsten fiber/metallic glass matrix (WF/MG) composite long rod projectile with different fiber diameters, the deformation and failure characteristics of various WF/MG composite long rod projectile were investigated systematically integrated with related penetration test results and mesoscopic finite element (FE) simulation analysis. The two-dimensional (2D) and three-dimensional (3D) meso-scale FE geometric models of WF/MG composite long rod projectile were established according to the actual distribution characteristics of metallic glass matrix and tungsten fibers. A coupled ‘thermo-mechanical-free volume’ constitutive model was applied to characterize the high shear sensitivity as well as high strength of metallic glass matrix. Moreover, the transformation of deformation and failure modes of projectiles under different impact velocities was analyzed in detail. Related analysis demonstrated that due to the differences in the mechanical properties among the tungsten fibers with different diameters, i.e., bending resistance and shear resistance, etc., the corresponding composite long rod projectile exhibit different failure characteristics during the penetration process, which further affects their penetration/perforation performance. The impact velocity also plays an important role on the deformation and failure modes of composite long rod projectile and the corresponding penetration/perforation performance. When the impact velocity is relatively low, the fibers in the nose of composite rod projectile exhibits unstable buckling phenomenon and gradually disperse during the penetration process, which makes the projectile nose blunt at a certain extent. This induces the increase in the penetration resistance and decrease in the penetration performance. As the impact velocity increases, small-diameter fibers reflux along the nose periphery while large-diameter fibers fail in shear. The large-diameter fiber further strengthens the self-sharpening behavior as well as the penetration performance of projectile. When the impact velocity and fiber diameter reach a certain upper threshold values, the thickness of ‘edge layer’ in the rod nose is sharply reduced, and thus the self-sharpening performance is weakened, and the penetration capability of projectile is decayed again. Related research is beneficial to predicting the penetration/perforation performance of WF/MG composite long rod projectiles with different fiber diameters under different impact velocities, and optimizing the structural design of projectile as well as the impact velocity, etc.