Abstract: The interaction of two arcs in the aluminum alloy twin-wire pulse metal inert gas (PMIG) welding effects the stability of welding process critically. The welding arc equivalent resistance contains more information of arc than the welding current or the arc voltage. So the equivalent resistance cross fuzzy entropy (CFuzzyEn) was proposed to evaluate the twin-wire PMIG welding stability. A series of experiments were carried out by using 3A21 aluminum alloy, and then the CFuzzyEn of arc equivalent resistance of these welds was calculated. The results showed that the more stable the welding process was, the greater the equivalent resistance CFuzzyEn would be. In addition, experiments with sudden changes of welding stabilities were designed, and then the equivalent resistance CFuzzyEn and fuzzy entropy (FuzzyEn) of these welds were calculated and compared. It indicated that the resistance CFuzzyEn was more sensitive to the change of welding stability than FuzzyEn and it could diagnose the point where the welding stability changed more accurately.