Ferrite number (FN) is a crucial parameter for austenite steel welding product, since it has significant influences on the crack sensitivity and ductility at cryogenic temperature. Normally, the FN is measured using a Feritscope or predicted according to WRC-1992 and the results generally match. This investigation detected the FN of 304L stainless steel multi-pass GTAW joint protected by mixed shielding gas with different N2 additions; the measured FN did not match with the FN predicted through WRC-1992 as the N2 content wasmore than 1%. The microstructure showed that a fully austenitized zone formed at the inter-bead of the joint shielded with Ar–1.0%N2 and Ar–1.5%N2, which resulted in the inaccuracy of the FN predicted through WRC-1992. The formation mechanism of the fully austenitized zone was proposed based on the nitrogen solubility and equilibrium phase transformation map. It is suggested that traditional prediction diagrams cannot give an accurate prediction of the FN during the multi-pass process shielded with N2 addition.