A simple mechanism for a vibration sensor based on fiber Bragg grating (FBG) is proposed. The mechanism utilizes the gravity force to allow the maximum sensitivity of the sensor into vertical direction while the minimum sensitivity for other two transverse directions. The sensor consists of an FBG cable and several masses to form a vibration system. The cable is extended to introduce a certain tensile force and acts as a spring. The sensitivity of the sensor is more than 1μstrain/gal for vertical direction. This simple sensor has been developed for damage detection of large structures. Several prototypes were fabricated. And extensive shake table tests were conducted to examine the performance. Because of the nonlinear behavior of the spring, it was found that the sharp peak will be elapsed beyond certain vibration level. This phenomenon is very useful to make the sensor as simple as possible without adding any damper element.