Abstract: In this paper, the non-contact air coupling method is proposed to measure the thickness of the structure. After the air coupled ultrasonic wave is transmitted to the test body, the frequency of the resonant signal is obtained by Fourier transform. The thickness of test body can be calculated from the relation between frequency and thickness of test body. Ultrasonic waves are transmitted from the air-coupled probe to the steel plate of different thickness. The thickness of the steel plate is measured by the resonance method, and the error is within 0.2 mm. The results show that the resonant frequency is different for different thickness test bodies. The defect thickness of different surface shape was detected accurately.