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The magnetic fields generated by the shielding current density are numerically investigated by use of spectral analysis. Coupled with the J-E constitutive relation, the governing equation of the shielding current density is numerically solved to obtain the time sequence of the magnetic fields. By means of the fast Fourier transform (FFT), the spectral analysis of the time sequence is performed. The results of the analysis show that the flux flow over the whole volume of the superconductor can be detected from the onset of the third harmonics of the magnetic fields.