Abstract: To solve the difficult problem of online nondestructive detection of delamination defects in concrete structures, a new method using air-coupled ultrasonic guided waves is proposed in this paper. Firstly the propagation characteristics of air-coupled ultrasonic guided waves in concrete structures are studied. Theoretical analysis and experiments show that a single A₀ mode can be excited when an air-coupled ultrasonic wave is incident on the concrete slab of 50 mm thick at 8.7°. Then, an experimental system of automatic scan test using air-coupled ultrasonic guided wave is constructed, and a pair of air-coupled probes tilted at an angle of 8.7° is used to excite and receive the guided wave signals on one side of the concrete structure, and it is found that the A₀ mode guided wave is sensitive to the delamination defects, and there is a monotonic change relationship between its amplitude and the size of the delamination defects in the scanning path. Based on this, the detection area is scanned, and two-dimensional imaging of the delamination defects is realized by the guide wave signal amplitudes at different positions. The experimental results show that this method can not only avoid the influence of coupling agent on the detection results, but also achieve the quantitative detection of the location and size of delamination in laminated concrete structures under service condition.