For composition design of amorphous and nanocrystalline alloys, prediction of their properties has been a long-sought goal. In this study, we reveal the quantitative relationship between composition and crystallization behavior in Fe-rich Fe B based amorphous alloys. Furthermore, we demonstrate that after using the mixing enthalpy as the threshold, the atomic size difference can be utilized as an indicator for amorphous-forming ability (AFA). By employing the correlations between composition and crystallization behavior/AFA, we developed new strategies for designing high- B s (saturation flux density) Fe-rich Fe-based amorphous/nanocrystalline alloys from thermodynamic, topological and comprehensive perspectives. These new strategies have the potential to be a universal guide on composition design for high- B s Fe-based soft magnetic amorphous/nanocrystalline alloys.