Abstract: Using the laser metal deposition (LMD) technology, 24CrNiMo alloy samples were prepared in Ar and air atmosphere, respectively. Optical microscope, scanning electron microscope, oxygen nitrogen hydrogen analyzer, microhardness tester, room temperature tensile test and impact test were used to investigate the microstructure and mechanical properties of the alloy prepared. The results showed that the morphology of the deposited solidified structure in Ar and air atmosphere were basically same, both were epitaxially grown columnar crystals, which changed into cell dendrites with decrease in the ratio of the interface temperature gradient (G) to the interface growth rate (v). The microstructures were all granular bainite. Heat accumulation temperature affected the size and distribution. The size and distribution of the particles were irregular because that the accumulated temperature in the Ar atmosphere was higher, while the shape of bainite in the air atmosphere was short rod-shaped and distributed in a certain direction; the hardness and tensile strength of the sample in the air atmosphere were slightly higher than those in the Ar atmosphere, but the plasticity and impact toughness of the former were slightly lower, mainly due to the presence of more oxide inclusions in the sample in the air.