Abstract: Based on the study of stability of aluminium MIG-welding in argon,the relation between the sensitivity of "intrinsic self-regulation" in meso-spray arc and the visible are length is investigated.The key points of designing "CC welding power source and constant speed wire feed" system are discussed.The short-compensator,the equation of centralized regulation,the optimum standard curves and the transfer function of this new type welding system are given,and a new thyristor controlled CAC welding power source for MIG-welding is made.By a series of process experiments and by comparison of this welder with CP and CC welding power sources,it is proved that this system is more advantageous to the MIG-welding of aluminium and its alloys.