In the welding of aluminum alloy large-scale structural parts,horizontal welding is the most difficult technique. How to reduce weld porosity defects is one of the current research hotspots. The pulsed MIG horizontal welding of 1561 aluminum alloy plates was studied in this paper. The effect of different welding process parameters on the porosity sensitivity was analyzed by X-ray detection and microstructure observation. The results show that the mixture ratio of helium and argon gas in a certain range,the high temperature preheating,the arc tilted upwards,are conducive to gas escaping from molten pool. With the increase of arc voltage and welding speed,it is beneficial to reduce the number of large pores in the weld,and with the increase of welding current,the number of large pores increases. This study can provide a solid experimental basis for the optimization of the welding process for horizontal welding of aluminum alloy.