Capacitive micromachined ultrasonic transducers (CMUTs) are regarded as an attractive candidate in bio-applications such as imaging and molecule monitoring. However, the previous researches on biochemical sensing are mostly air-coupled application based on a CMUTs array because cell-to-cell mutual radiation and large motional loss in liquid environment are able to produce nonignorable noise. For a CMUTs cell, its characteristics (e.g., electrical, mechanical, and acoustic) are susceptible to parasitic effects owning to the physically capacitive dielectric dispersion, motional damping, and connection loss. Neglecting the parasitic effects on multidomain characteristics of CMUTs leads to significant robustness errors. Furthermore, finite element method (FEM) is not sufficient enough to model such parasitic effects when considering an integrated circuit interface and the evaluation of system responses. This article highlights a lumped element model (LEM) to analyze the behaviors of a label-free biosensor based on a single CMUTs cell directly operating in liquid. We successfully explore the performance of the biosensor with different types of parasitic effects in transient and frequency domains through LEM and FEM. The parasitic effects on mass loading of sensing layer and biomolecule such as deoxyribonucleic acid (DNA) and Immunoglobulin G (IgG) probes are predicted by LEM based on their surface mass densities. The proposed approach is capable of analyzing the variation's margin of CMUTs-based biosensors to improve robustness for parasitic extraction in liquid.