Herein, 18Ni-300 maraging steel parts were printed using selective laser melting process to examine the influence of positive defocusing distance on the printing quality and mechanical properties of 18Ni-300 forming parts. Results show that the power density is too high for a small defocusing distance (+1.5 mm), causing turbulence in the molten pool and obvious spheroidization is observed on the molten channel surface. Moreover, large pore defects appear inside the formed sample, the mechanical properties deteriorate, and the tensile fracture shows quasi-cleavage fracture characteristics. With an increase in the defocusing distance, the power density decreases and the spot and melting channel become wider, which improves the properties of the samples. Thus, the tensile fracture mechanism transforms into plastic fracture. For +2.5 mm defocusing distance, the power density is moderate, weld channels are well formed, internal structure is uniform, and interlayer bonding is enhanced. The hardness and tensile strength are 37.7 HRC and 1215 MPa, respectively. When the defocusing distance increases to more than +3 mm, the overlap ratio between the melt channels is too large and the over stacking phenomenon occurs. Furthermore, the power density is too low and the spot penetration force is weakened, thereby affecting the interlayer bonding and decreasing the sample performance.