Abstract: A new intensity modulated optic microphone theory model was introduced. Single-mode optical fiber here was used to realize collimation of beam. The propagation law in collimation lens was discussed, and light intensity modulation using matrix optical principle and Gaussian coupling theory was realized. The system contains MEMS pressure vibration membrane to pick up vibration displacement. Through the analysis of membrane size and the change of pressure, the related parameters about the sensitivity of fiber optic displacement sensor could be acquired. Simulation results show that the proper choice of parameters can make the sensitivity of sensor improve in orders of 103 magnitude than traditional light intensity modulated methods in condition of no increasing the complexity of structure.