If bulk acoustic wave (BAW) resonator under mass loading, an mass loading equivalent inductor will be added in Butterworth Van Dyke (BVD) circuit model of the resonator. In order to get the quantitative relation between the inductor and the mass, so that we can carry on the system level behavior simulation of BAW sensor, the comparative modelling method between the 3D multiphysics simulation and the equivalent circuit simulation is proposed. The quantitative relation between mass loading equivalent inductor and the mass is got by comparing the results of the 3D multiphysics simulation in COMSOL Multiphysics software and results of the equivalent circuit simulation in the ADS software. Instanced with a film bulk acoustic resonator (FBAR) case, detailed application procedures of the method is introduced. Accordingly, quantitative relation in this case is that the effect of 1 nH added in mass loading equivalent inductance is equal to the effect of 0.1 ng added in mass. At last, the quantitative relation are used in BAW sensor detection circuit which based on Pierce oscillator for the system-level behavior simulation, the results show that the decrease of oscillation frequency is 6 MHz due to the increase of per 1 nH in mass loading equivalent inductance, i.e. the change of the oscillation frequency and the equivalent circuit simulation results are identical with each other. Accordingly, the results validated that the detection circuit can be used for BAW sensor frequency signal detection. The quantitative method is also applicable to quartz crystal microbalances (QCM).