Following the random magnetic anisotropy model proposed by Alben for amorphous ferromagnets and taking into consideration the two-phase microstructure of nanostructured soft magnetic alloys, we propose a two-phase random magnetic anisotropy model for nanostructured soft magnetic alloys. We obtain the relationship of the effective magnetic anisotropy density with the microstructure parameters and magnetic parameters of ferromagnetic phases in the alloy, and also the dependence of the magnetic propertics on the effective magnetic anisotropy density. The experimental results of X-ray diffraction and magnetic measurements of Ｆｅ７３.５Ｃｕ１.０Ｎｂ３.０Ｓｉ１３.５Ｂ９.０ nanostructured soft magnetic alloys give favourable support to the above theoretical research.