In this work, we demonstrate a behavioral PSPICE model for silicon nanowire (SiNW) field-effect transistor (FET) biosensors, which is suitable to simulate frequency domain electrical measurements. The model is divided into two separated components: an electrochemical part for the liquid/solid interface at the gate input and an FET part simulating the SiNW characteristics. In our study, the parameters of the FET model are obtained from characterization measurements of real devices, which are fabricated in our research group. Measurements were performed with a self-developed readout system. The model can be used to investigate the effects of solution conductivity and the drain and source capacitances of the SiNW sensor. We observed that the impedance spectra consist of a low-pass filter domain at low frequencies and a resonance domain at higher frequencies. The former is mainly influenced by the source capacitance and the oxide/liquid interface of the devices, whereas the latter is mainly dominated by the drain capacitance, in case that the solution conductivity is kept constant. In addition, based on the relationship of the solution conductivity and the experimental data, we suggest a value called representative resistance of the solution. In this study, the model was compared to real spectra and we achieved good agreement between simulation and experiments. Our model provides a clearer view for frequency domain measurements with SiNW sensors, offers the possibility to optimize our SiNW design and forms the basis to include and explain sensing events in future biomedical assays.