Abstract: This paper analyzed and discussed the problem of porosity defects when MIG automatic welding is used to weld standard metro aluminum alloy roof profiles.By using the method of controlling variable,the welding current,arc voltage and welding speed are kept consistent,and the size of the protective gas flow and the swing of the welding gun are changed respectively.The weld formation and weldability analysis are carried out on the welding seam after welding,and the penetration detection is carried out to observe and study the porosity defects.It is found that when the welding current,arc voltage and welding speed are controlled,and the shielding gas flow is less than 22L/min,the weld is poorly formed and there are more pores.When the shielding gas flow rate is between 25L/min and 28L/min,the weld formation is good,and the visible porosity is obviously reduced,and the porosity defect is basically disappeared after penetration inspection.When the shielding gas flow is too large,the protection effect of the molten pool decreases,the arc temperature is high,and the risk of damaging the base metal is high.Control welding current,arc voltage,welding speed and protective gas flow to make the welding gun swing,the porosity generated by the welding gun swing is significantly less than the porosity generated by the welding gun without swing.