We put forward an physical concep for the mesoscopic impedivity of Fe-based nanocrystalline alloy for the first time, and its formula ρ=-μ{A}/{t}/Δ×His obtained by using Maxwell system of equations. The formula shows that the mesoscopic impedivity ρ of Fe-based nanocrystalline alloy depends on the mesoscopic magnetic field intensity H, mesoscopic magnetic vector potential A, and mesoscopic permeability μ. The mesoscopic magnetic vector potential A is caused by the effect of quantum mechanics with the mesoscopic structure in the studied material, and is determined by the microcosmic structure of the studied material. Magneto-resonance frequency shift of nanocrystalline Fe735Cu1Nb3Si135B9 powder for its giant magneto-impedance(GMI) effect and frequency resonance magnetic field intensity shift of nanocrystalline Fe730Cu10Nb15V20Si135B90 microwire with the multilayer structure for its GMI effect are explained successfully by the formula.