Research is conducted on a micro-electromechanical system （MEMS） fiber optic acoustic sensor by combining an MEMS diaphragm and ultra-small graded-index （GRIN） fiber optic lens， and a performance test method of the MEMS fiber optic acoustic sensor based on the swept-source optical coherence tomography （SS-OCT） demodulation system is investigated. The fabrication process of the MEMS optical fiber acoustic sensor combining the MEMS diaphragm and ultra-small GRIN optical fiber lens is designed and studied on the basis of modeling and analysis， and an acoustic and vibration measurement system based on SS-OCT is constructed. The performance test and calibration of the sensor are performed by conducting measurement experiments of a single-frequency acoustic signal， a mixed-frequency acoustic signal， frequency response， sound pressure sensitivity， and system stability. The results show that the frequency response range of the studied MEMS fiber optic acoustic sensor sample is 50 Hz-4.5 kHz. At a frequency of 300 Hz， the sound pressure sensitivity of the sensor is 21.63 nm/Pa， signal-to-noise ratio is 44.1 dB， linearity is 98.97%， and repeatability standard deviation is 0.003. The studied MEMS optical fiber acoustic sensor is shown to be feasible， and its sensing performance can be effectively measured by using the SS-OCT demodulation system.