In this paper a new fiber-optic Microelectromechanical System (MEMS) acoustic sensor system for gunship acoustic detecting is developed. A novel MEMS acoustic sensor used in the system is designed. The sensor is based on extrinsic Fabry-Perot (FP) interferometer coupled to an external membrane for modulating the FP cavity length and a light--emitting diode (LED) as light source. The relation expression between displacement of cavity length and light intensity is deduced. Theoretical signal of the sensor is simulated.