The miniaturization and mobility of nuclear reactors have become an important trend in the development of nuclear energy. In order to simplify the design of shielding materials with improved complexity and reduced weight, 3D B4C‐W‐based composites were fabricated via fused deposition molding using highly‐filled granular feedstocks containing 62 vol% B4C‐W powders (boron carbide accounted for 30 wt% and tungsten for 70 wt%) and 38 vol% polymer binders (60 wt% Carnauba wax, 22 wt% polypropylene, 13 wt% polystyrene, and 5 wt% stearic acid). The rheological properties and microstructure of the feedstock and extruded filaments were clarified. Roles of the printing parameters including extrusion temperature, platform temperature and deposited‐layer height in the morphology of 3D composites were investigated in detail. Extruded filaments with good shape retention, dense fracture surface, and uniformly dispersed B4C‐W grains were achieved, benefiting from the smooth printing and shear thinning behaviour of the feedstock. Defects including warping, small pores or stacking pores could be formed under improper printing parameters, owing to the poor bonding strength between deposited layers induced by thermal internal stress or decomposition of wax. 3D composites with large size of 130 × 130 × 5 mm were fabricated, which showed a satisfactory compressive strength of 34.8 MPa. This work showed a facile route to fabricate 3D radiation shielding materials based on highly‐filled polymers.

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