The porosity in the welded seam can be generated easily during the CO₂ laser nonpenetration deep welding of low carbon steel, which affects the quality of welding. This research uses the mild steel as the object for the high quality requirements of welding. The advanced high-power CO₂ laser generator was used for the welding experiment. The method of cutting cross-section of weld seam was used to analyze the porosity number and observe the morphology and location of porosity in the weld. The effects of such process parameters as shielding gas flow, laser beam inclination, laser power and welding speed on porosity generating were discussed. The research results show that the generating of porosity is due to the unstable collapse of the keyhole in the process of CO₂ laser nonpenetration welding of low carbon steel. The porosity would be formed when the speed of bubble escaping form the weld pool is lower than the speed of melting metal solidifying. The results also show that with the increase of shielding gas flow, the porosity number presents a curve of increase firstly and then decrease. The lowest porosity number can be obtained at a 35 L/min of gas flow. With the increase of laser beam inclination angle, the porosity number shows a trend of decrease after increase. Under the condition of deeper penetration welding, the relatively lower porosity number can be obtained at the inclination angle of 22.5°.When the laser power is 4 kW, the porosity number is lowest. At the condition of lower welding speed, the bubble can escape easily for the longer existence period of melting weld pool. Thus lower porosity number and porosity number can be achieved. The porosity can be inhibited effectively at a welding speed of 0.75 m/min.