The effects of key process parameters (laser power, scanning speed, and laser energy density) on the defects of 316L stainless steel samples are studied by experimental observation and numerical simulation. The results show that porosity is the main reason for the decrease in density. The generation of different types of pores is closely related to the process parameters, especially the laser energy density. Insufficient laser energy input (E<52.778 J/mm³), especially insufficient molten pool width, is the key cause of unfused porosity. The formation of keyhole is mainly caused by high temperature and deep size of molten pool due to high laser energy input (E≥93.056 J/mm³). It is difficult to eliminate stomata completely, but reasonable control to avoid excessive laser energy input is beneficial to reduce stomata size. By adjusting the process parameters reasonably, the density can reach 99.62%.