This study aimed to modulate the magnetic properties of selective laser melting NiFeMo alloy. This was achieved via the controlled addition of Mo elemental in-situ alloying content to change the tissue morphology and crystal structure of SLM-formed (NiFeMo)100−xMox alloy (x = 0 wt%, 0.25 wt%, 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt%, 1.5 wt%, 2.0 wt%). The results of scanning electron microscopy (SEM), X-ray diffraction, electron backscatter diffraction (EBSD), DC B-H hysteresis tester, and transmission electron microscopy (Titan-G2) indicate that all samples are predominantly face-centered-cubic (FCC) γ- (Ni, Fe) solid solutions and typical soft-magnetic hysteresis line features. However, the lattice parameter a of γ- (Ni, Fe) decreased and then increased with increasing Mo content. When the added Mo content is 0–1.25 wt%, the Mo particles inside the (NiFeMo)100−xMox alloy become nucleation sites, creating a diffusive metallurgical behavior with the Ni and Fe elements, and preventing the growth of columnar crystals, leading to the reduction of LAGBs density and the enhancement of the {111} texture orientation in the easy magnetization direction, increasing Bs and the decrease of Hc of the NiFeMo alloy. At the Mo content of 1.25 wt%, the soft magnetic properties were optimal, with Bs and Hc being 0.704 T and 17.31 A/m, respectively. Conversely, when the added Mo content was 1.5–2.0 wt%, the solubility of Mo elements in the γ- (Ni, Fe) solid solution was limited, which resulted in the occurrence of metallurgical defects within the organization such as unfused Mo particles, unfused areas, and porosity. Moreover, when the added Mo content increased to 2.0 wt%, the {111} lattice spacing in the easy magnetization direction increased by 0.2261 nm compared to the NiFeMo alloy, and the average lattice distortion Δε increased to 0.16%, causing a decrease in the soft magnetic performance of the (NiFeMo)98Mo2 alloy, with Bs reducing to 0.6666 T and Hc increasing to 23.44 A/m. •The microstructure evolution of NiFeMo alloy formed by in-situ alloy-SLM was analyzed and described in detail.•The crystallographic relationship of NiFeMo alloy with Mo element was analyzed on an atomic scale by HETEM.•Analyzed the changes in soft magnetic properties of NiFeMo alloy with different Mo additions.•Clarified the influencing mechanism of Mo element in-situ alloying on the magnetic properties of NiFeMo alloy.