The invention provides a nanocrystalline magnetically soft alloy, an amorphous magnetically soft alloy and preparation methods of the nanocrystalline magnetically soft alloy and the amorphous magnetically soft alloy. The nanocrystalline magnetically soft alloy comprises the compositions of Fe, Cu, Si, B, and Nb, and further comprises at least one element between Mn and Cr, wherein the atomic percentage content of each element at the at least one between the Mn and the Cr is not more than 3.5 at%. The antiferromagnetism of the Mn element and/or the Cr element is utilized, and a heat treatment mode of multi-stage rate annealing is combined, so that the high-frequency magnetic permeability of the magnetically soft alloy is improved, the high-frequency loss of the magnetically soft alloy is reduced, and the high-frequency magnetic performance of the magnetically soft alloy is improved; meanwhile, the crystallization temperature interval is narrower, and the heat treatment temperature interval is wider, so that the high-quality nanocrystalline magnetically soft alloy or the high-quality amorphous magnetically soft alloy are obtained favorably and the preparation cost is reduced; and inaddition, the oxidation resistance of the Mn element and/or the Cr element is further utilized, so that the tolerance of impurities is improved, the surface of an amorphous strip is prevented from being crystallized, the amorphous forming capacity is improved, and the high-quality magnetically soft alloy can be prepared through low-purity commercial raw materials.