Abstract: M42 high-speed steel is a key material for manufacturing high performance complex precision cutting tools. Additive manufacturing demonstrates significant advantages in forming tools with complex external geometries and internal functional flow channels. This study successfully fabricated M42 high-speed steel using electron beam powder bed fusion （EB-PBF） additive manufacturing technology. The effects of energy density （E） on the forming quality, microstructure, hardness, strength, and toughness were systematically investigated. The results show that when the E value is in the range of 40 to 50 J/mm³, the relative density of the specimen reaches above 99.5%. The microstructure mainly consists of an equiaxed ferrite matrix with a grain size of 5.4 to 6.3 μm, a micron-scale network of eutectic carbides （fine rod-like M₆C type and short rod-like M₂C type）, and dispersed ellipsoidal or short rod-like secondary carbides of M₂C type with a size of 50 to 200 nm. The high carbon and high alloy content leads to solute segregation even under rapid cooling conditions, which is the main reason for the formation of the network eutectic carbides. As the E value increases from 35.0 J/mm³ to 54.5 J/mm³, the cooling rate of the melt pool decreases, and both the thickness （0.12 to 0.31 μm） and the area fraction （12.5% to 18.6%） of the eutectic carbides increase accordingly, while other microstructural features show no significant change. The hardness of the high density specimen is about 66HRC, which is slightly lower than that of quenched and tempered wrought material. However, due to strain mismatch and stress concentration caused by the network eutectic carbides, both the bending strength （2 220 MPa） and the V-notched impact toughness （4.1 J/cm²） are lower than those of the wrought material. This study further indicates that optimizing the morphology and distribution of carbides through post heat treatment or interlayer remelting during the forming process is expected to improve the strength and toughness of the material.