Metallic glasses (MGs) possess the merits of high strength and a large elastic limit. However, they suffer from little tensile ductility in the inhomogeneous deformation regime due to strain softening and shear localization. In this work, we report a substantial increase in tensile plastic strain (ε_p) from 2.8% to 10% in a Fe-based metallic glass (MG) via non-affine thermal strain (NTS), accompanied by a significant intensity enhancement and a considerable decrease in activation energy (32%) of the β-relaxation. Notably, pronounced strain hardening is observed during tension. These extraordinary tensile properties are structurally attributed to the NTS-promoted formation of a chemical-fluctuation-mediated network structure consisting of interconnected Fe-rich medium-range orders (MROs) and surrounding metalloid-rich clusters, as well as the subsequent temperature and stress-induced unique evolution of the multiscale structural heterogeneities. Specifically, the stress-induced unique MRO formation, α-Fe nanocrystallization, and the irreversible relaxation-induced structural ordering jointly interact with shear banding to transform strain softening into hardening, leading to excellent ductility. These findings demonstrate that simultaneous relaxation-assisted and transformation-mediated deformation stabilizes the inhomogeneous plastic flow under tension, overcoming the ductility bottleneck of MGs.