Abstract: The forming conditions of stable keyholing puddle in the aluminium alloy plate with the thtickness of 6.0mm during vertical-up A.C. PAW process has been studied in this paper by the SYNCROWAVE-300(s) welding power source associated with the welding current and plasma gas flow controllers. It shows that the stable keyholing process and the better keyholling puddle can be acquired by incresing plasma gas flow or welding current, but only through increasing both welding current and plasma gas flow can the result be the best. The reasons of forming different keyholing puddles and the modes of the different keyholes have been also presented in the paper.