316L stainless steel parts were manufactured on TB-100 selective laser melting equipment with independent intellectual property rights. The low-magnification defects of forming parts with different process parameters (laser power, scanning speed and scanning distance) were studied by metallographic microscope, mainly air bubbles, pores, microcracks and macrocracks. The formation mechanism of various kinds of defects were analyzed. the microstructure and composition of the defects were studied by means of SEM and EDS. The results show that the pores are mainly circular air pores, irregular process pores and oxide inclusions. The air bubbles are mainly hydrogen bubbles and nitrogen bubbles, while the microcracks are mainly caused by penetrating pores and thermal cracks caused by excessive internal stress, macrocracks are mainly the layer cracks caused by excessive residual stress, which belong to cold crack. Linear energy density (E＝P/v) is introduced as a synthetic parameter. As the line energy density increases, the pores decrease gradually. However, when the linear energy density reaches 400 J/m, cracks appear, and a large number of air bubbles defects appear. As the linear energy density continues to increase, the cracks increases gradually. When the linear energy density reaches 583 J/m, the defects were mainly cracks and bubbles decrease. When the linear energy density reaches 875 J/m, the cracks are strip-shaped and the size is significantly increased, and cracks are connected to each other. According to the analysis and test verification, the optimal process parameters of 316L stainless steel are laser power 190～210 kW, laser speed 800～1 000 mm/s and scanning interval 0.90～0.11 mm, and the linear energy density is about 200 J/m. There are no cracks, basically no bubbles, a small amount of pores, and the product density reaches 99.7%.