In order to utilize non-inert casting atmospheres to improve the glass forming ability (GFA) of Fe-based bulk glassy alloys, in this work, dependence of GFA, composition and soft magnetic properties on casting atmosphere species is observed for Fe77Mo2P10C4B4Si3 glassy alloy. Firstly, it was found that GFA can be significantly improved by some non-inert atmospheres, as when cast under N2/air/O2, its critical diameter can reach to 3.5 mm, which is larger than that of sample cast in inert Ar. Moreover, the component contents on the alloy surface vary greatly with the casting atmospheres while the whole bulk composition remains almost unchanged. In addition, this Fe-based glassy alloy exhibits identical soft magnetic properties regardless of the applied casting atmospheres. The mechanism for the positive effect of non-inert atmospheres on the GFA of Fe77Mo2P10C4B4Si3 glassy alloy may be involved with the surface properties of alloy liquid such as surface tension and the modified superficial composition. •Dependence of GFA on casting atmosphere species is observed for a FeMoPCBSi alloy.•Its Dcr is improved to 3.5 mm by replacing Ar with non-inert casting atmospheres.•The component contents on alloy surface vary greatly with the casting atmospheres.•These alloys exhibit identical magnetic properties despite of atmosphere species.•The mechanism may be involved with the surface properties of alloy liquid.