A fiber grating ultrasonic sensor based on coupling cone structure is proposed. Numerical simulations and optimization of the ultrasonic coupling cone structure are performed, and the influence of fiber-grating length on the ultrasonic detection performance is analyzed. In conjunction with edge-filter-demodulation technology, the proposed ultrasonic sensor is applied to the identification of defects in a 6061 aluminum alloy plate. The experimental results show that the proposed ultrasonic sensor can effectively detect the Lamb wave transmitted in the aluminum alloy plate. By analyzing the variations in amplitude of the temporal waveform and S₀ mode of the Lamb wave, hole-type defects and defect size in the aluminum alloy plate can be readily determined. The proposed ultrasonic sensor has a number of advantages, including high sensitivity, flexible and adjustable positioning, and reusability, all of which are key factors for its potential application in the field of structural damage identification.