Nuclear power plant (NPP) accidents can cause severe effects. In order to ensure the normal operation of NPP, instrument and control system (I&C) composed of multiple sensors plays an important role in it. The sensor consists of sensitive components and functional circuits. In this paper, temperature sensor commonly used in NPP, PT100, is used as an example, and a bottom-up based physical analysis method is used to review the degradation mechanism and physical model of key components in resistance temperature detector (RTD) circuits. Resistors, capacitors, and MOSFETs are the key components of sensor circuits. Thin film resistors are the most widely used resistors due to their good performance. The transformed Arrhenius’ equation is used to describe its degradation characteristics. For the most commonly used aluminum electrolytic capacitors, the model reviewed in this paper can accurately describe the changes in equivalent series resistance (ESR), and use this as a criterion for determining capacitor failure. MOS technology is widely used in analog circuits. We have summarized three physical degradation processes, channel hot carrier (CHC), negative bias temperature instability (NBTI), and time-dependent dielectric breakdown (TDDB), that affect MOSFET performance. The physical model of the key component degradation of the sensor circuit summarized in this paper provides a basis for the subsequent establishment of a circuit-level degradation model.