In order to study the characteristics of porosity formation of mild steel under two conditions of penetration and non-penetration, mild steel (6 mm and 8 mm thick) was welded with a welding power of 3 100 W and a welding speed of 10mm/s, respectively, under the control of two parameters which is shielding gas and surface oxide. The method of cutting cross-section of weld seam was observe the morphology and location of porosity in the weld by scanning electron microscope(SEM).EDS point scanning analysis in the wall of porosity and discuss the causes of porosity formation. In addition, the permeability experiment was designed to study the influence of laser power and welding speed on the porosity. The research results show that the number and rate of porosity in the condition of penetration are far less than those in the condition of non-penetration. The shielding gas of CO2 and the surface oxides can inhibit the pores in the welding process. Under the condition of non-penetration, the porosity formation in the weld are metallurgical pores and process pores. But in the condition of penetration, the pores of the weld are round H2 pores. The content of chemical elements of metallurgical pores in the porosity and weld zone is greatly different, which is mainly caused by metallurgical reaction in the molten pool. Moreover, The higher the laser power, the less the number of porosity, but the rate of porosity first decreased and then increased. With the increase of weld velocity, the number of porosity increased, but the rate of porosity decreased first and then increased.