The 4Cr5MoSiV1 die steel samples are fabricated by the selective laser melting (SLM) technique. The influence of laser linear energy density η on microstructures, carbon loss and micro-hardness of samples is investigated. The results show that the microstructures of SLM-formed 4Cr5MoSiV1 die steel samples are mainly martensite and a little amount of residual austenite. The decarburization reaction, splashing behavior and elemental ablation together cause carbon loss, and the carbon loss rate is as high as 17.7% at η=950 J·m-1. With the increase of η, the grain sizes of samples increase, the carbon loss rate increases and the martensite content decreases. Under the same η, the grain size and the carbon loss rate in the transition zones of samples are the smallest, while those in the heat affected zones are the highest. With the decrease of η, the pore defects become more. When η is too large, the cold cracks appear. When η is 905 J·m-1, the samples possess uniform microstructures, high densities, and relatively high micro-hardness (that in central zone of molten pool is 710.3 HV and that in transition zone is 732.4 HV).