For a piezoelectric (PZ) immunosensor operating under damping loading conditions, the mass loading and damping loading effects cannot be distinguished by frequency measurement alone. In this work, a special relationship between the vibrational amplitude on the surface of a quartz crystal and the viscosity and density product of the tested liquid is derived using the energy transfer model (ETM). To detect the NuTu-19 antibody concentration in immune serum, we simultaneously measure the resonant frequency and rectified load voltage of the PZ immunosensor. Excellent agreement is obtained between the ETM-calculated value and the actual value obtained by the Bradford method. The experimental results proved the correctness of the deduced equation and showed that the presented analytical method laid a foundation for further development of all types of quartz crystal sensors in the liquid phase.