This paper proposed a fiber Bragg grating (FBG)-based three-axis vibration sensor by using only five sensing elements. A composite structure of the cross-beam-type elastomer with multiplexed FBGs has been designed for three-axis vibration sensing. Because of the mechanical decoupling function of the structure and the reasonable arrangement of FBG sensing elements, the designed sensor possesses great advantages of low cross coupling and self temperature compensation capability. Theoretical model of the composite structure has been established, strain distribution and dynamic characteristics of the composite structure have been studied by the finite element analysis method. Amplitude-frequency test result shows that resonant frequencies of the sensor in X, Y, and Z directions were 980Hz, 960Hz, and 640Hz, respectively. Vibration sensitivities in X, Y, and Z directions were 9.7pm/g, 10.13pm/g, and 7.45pm/g, respectively. Vibration calibration test have validated that the designed sensor possess the capability to measure three-axis vibration.