Intensity modulated fiber optic sensor is one of the fastest growing and most widely used fiber optic sensors. A double optical-fiber vibration sensing system with a new kind of membrane is introduced in this article. The system is based on optical intensity modulation and is characterized by the membrane that is used to sense vibration. The mechanism of film sensing is described and the model showing the relationship between film parameters (reflection coefficient and radius of curvature) and the sensor performance is established. With the simulated responses, a membrane structure with high sensitivity is developed. The experimental setup used to test the performance of the sensor is presented. The sensor can detect the minimum vibration amplitude of 0.02 mu m. The frequency response is from 300 Hz to 1.8 kHz.