To address the dependence of peak frequency on driving amplitude and enhance the frequency stability of nonlinear resonant systems, a novel mass sensor based on the mechanical dual-frequency locking phenomenon has been designed. Initially, a three-degree-of-freedom magnetically coupled model was constructed to theoretically analyze and predict the dynamic behavior of coupled model. Subsequently, the mechanical dual-frequency locking phenomenon was experimentally verified, and the detection principle of the mass sensor was proposed. Additionally, the influence of coupling spacing on the first frequency locking, second frequency locking, detection range, and linearity was investigated. Experimental results demonstrated that the resonant system exhibits relatively stable peak frequencies within two distinct driving voltage intervals. Specifically, the first frequency locking was observed at a driving voltage of 60~105 V, with a stable frequency around 27.18 Hz. The second frequency locking appeared at a driving voltage of 120~150 V, with a stable frequency around 27.61 Hz, and a frequency shift jump of 0.43 Hz occurred between these two stable ranges. The detection of adsorbed mass was achieved by monitoring the first frequency-locking range and the associated unlocking position, combined with the corresponding frequency shift. By appropriately adjusting the coupling spacing, the detection range of the sensor for quality has been increased from 4 mg to 5 mg, and the sensitivity has increased from 0.09 Hz/mg to 0.12 Hz/mg. The conclusions drawn enhanced the peak frequency stability of the sensor and offered a new possibility for mass sensors.