Abstract: Micro damage in composites usually leads to catastrophic failures in the following applications and therefore needs to be detected. However, it is difficult to detect transverse cracks in composite laminates by using ultrasonic nondestructive testing. In this research, a highly-sensitive and broad bandwidth phase-shifted fiber Bragg grating ultrasonic sensing system was developed and applied to detect Lamb wave propagating in a cross-ply carbon fiber reinforced polymer composites. After conducting data process on the detected Lamb wave, it is found that along the increase of crack numbers induced by the standard three-point bending test, the amplitude of the waveform and the peak value of the spectrum both linearly decrease. After compared with conventional lead-zirconate-titanate sensor, all research results demonstrate that the Lamb wave detected by phase-shifted fiber Bragg grating sensor shows a higher declining rate when the number of matrix cracks increase. Thus, the novel optical-fiber-based sensor succeeds in detection of weak ultrasonic signals with middle frequency of 300 kHz, and can precisely evaluate the micro transverse cracks in composite laminate.