Abstract: Multilayer foam bonding composites contains a large number of porous/honeycomb structures, making it difficult to achieve effective ultrasonic coupling and incidence when conducting non-destructive testing with conventional ultrasonic techniques. This results in ultrasonic detection of internal defects becoming a common industry challenge. This paper employs air as the ultrasonic coupling medium for non-destructive testing of multilayer foam bonding composites, and determines key process parameters based on measured data from this type of composite. The key detection parameters of air-coupled ultrasonic technique (ACUT), such as probe spacing, pulse string number, average count, center frequency, preamplification ratio, and filtering frequency, are selected as the research variables. The influence laws of each parameter on the amplitude of defect echo signals are discussed, and the optimized parameters suitable for ultrasonic testing of multi-layer foam bonding composites are determined: the probe spacing is 250 mm, the pulse string number is 5, the average count is 1, the center frequency is 140 kHz, the preamplification ratio is 20%, and the filtering frequency is 80-200 Hz. By preparing multilayer foam bonding composite defect samples with different depths and sizes at the interface of functional materials, the air-coupled ultrasonic technology with optimized parameters is used for detection. The results show that the maximum relative errors of detection results are 7.50%, 5.00% and 4.00% for prefabricated circular defects with diameters of 20, 30 and 50 mm, respectively. This confirms that the air-coupled ultrasonic technique can achieve quantitative detection of interfacial debonding defects at different depths inside this type of composite.