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This paper investigates the effect of the SNR of the magnetic response harmonic signal on the sensitivity of biomolecular detection. A method is proposed for measuring the 2nd and 3rd harmonics of magnetic nanotags under a DC bias field, avoiding the influence of antimagnetic and paramagnetic substances around the magnetic nanotags, as well as the background magnetic field, on the 1st harmonic signal. Specific streptavidin-modified magnetic nanoparticles were used as measurement objects, and their binding to biotinylated polystyrene microspheres was measured by a prepared detection device to simulate the detection of specific biomolecules. Using the 2nd to 3rd harmonic ratio, the lower limit of detection of the lowest 4amol (0.4pM) polystyrene microspheres in a 100uL solution was eventually determined.