Based on the AFM observation of the mesoscopic structure and the XRD experimental results of Fe73.5Cu1Nb3Si13.5B9 nanocrystalline alloy, we proposed a model for exploring the influence of the mesoscopic structure on the soft magntic properties of this kind of Fe-based nanocrystalline alloys, and calculate the frequency function——D-function for the soft magntic properties of Fe73.5Cu1Nb3Si13.5B9 nanocrystalline alloy. With the frequency function, we succeeded in exploring the influence of the frequency on the soft magntic properties of Fe73.5Cu1Nb3Si13.5B9 nanocrystalline alloy. Analysis shows that the D-function is a complex function, the real part Re(D) shows the inductance and capacitance of nanocrystalline Fe73.5Cu1Nb3Si13.5B9, the imaginary part Im(D) shows its resistance. We have built an equivalent parallel RLC model for the mesoscopic structure of Fe73.5Cu1Nb3Si13.5B9 nanocrystalline alloy. Based on the model we have calculated vextGMI=v|Re(D)=0, which is the condition of maximum GMI value for Fe73.5Cu1Nb3Si13.5B9 nanocrystalline alloy, as well as influence of the factors μ,σ,ω,R,Hex and the micro-magntic-structure on the maximum GMI value.