In view of the possible inclusion of Staphylococcus aureus in dairy products, a magnetoelastic S. aureus wireless sensor was developed to study the detection the mechanism of its magnetoelastic sensor in dairy products. By comparing the magnetic resonance wireless sensor's resonance frequency shift and fluorescent experiment in bacteria and aseptic dairy liquids, the analysis results showed that the resonant frequency of the magnetoelastic sensor could better reflect the weight of the sensor. When the mass of the sensor decreases or the liquid increased, the resonance frequency of the sensor decreased. It was also confirmed that the viscosity of the liquid was significantly suppressed by the viscosity of the magnetoelastic sensor. Even though the viscosity changed only 0.15 mPa·s, the resonance frequency shift could reach 0.5 kHz.