This study investigates the impact of filling density (50%, 75%, and 100%) on the dynamic behavior of carbon-fiber-reinforced PETG (CF-PETG) composites fabricated using the cost-effective fused filament fabrication (FFF) technique, commonly employed for producing complex structures. The primary objective is to evaluate how different filling densities influence the material’s dynamic characteristics, such as compressive strength, modulus, and damage resistance. Dynamic compression tests were conducted using a split Hopkinson pressure bar (SHPB) at various strain rates, with input pressures of 1.4, 1.7, 2, and 2.4 bars. A high-speed camera was used to monitor and record the damage evolution, offering insights into fracture mechanisms in 3D FFF-printed CF-PETG composites. The results demonstrate that increasing the filling density significantly improves the dynamic characteristics, leading to enhanced compressive strength, higher compressive modulus, and better damage resistance. Higher filling densities result in a more compact and interconnected structure, which increases the stiffness and overall strength of the composite.