This study presents the development of a novel MEMS-based Fabry-Perot accelerometer with a composite cavity， designed for high-sensitivity， small-range vibration monitoring in the extreme conditions （500°C， 2 MPa） of lead-bismuth reactors. To optimize the sensor's performance， a detailed analysis was conducted using a mechanical model of a cross-beam and mass block structure to evaluate the influence of structural parameters on sensitivity， measurement range， and resonant frequency. This analysis guided the determination of optimal design parameters for the sensitive structure. The chip structure， featuring a composite cavity， was fabricated through a precise double-sided bonding process， ensuring the achievement of the required mechanical properties and thermal stability. Following fabrication， the sensor was assembled and manufactured to conform to stringent design specifications. A dedicated testing platform was established to evaluate the sensor's dynamic characteristics， sensitivity， operational range， and thermal performance under high-temperature conditions. Experimental results demonstrated the sensor's robust functionality， maintaining performance at temperatures up to 500 ℃， and achieving a high sensitivity of 7.69 nm/g within a frequency range of 2-30 Hz. The device provides a measurement range from -11g to +11g with a resolution of 0.04g， enabling precise detection of flow-induced vibrations critical for wear monitoring in reactor components. These attributes enable effective assessment of the wear state of fuel assemblies， a key factor in reactor safety and operational reliability. The accelerometer's high sensitivity and operational reliability underscore its potential as an advanced tool for early detection of vibration-induced component wear in extreme environments. This development marks a significant contribution to improving reactor safety and efficiency， offering a powerful solution for proactive maintenance and monitoring in lead-bismuth reactors.