Abstract: In single-sided infrared thermography, thermal waves must propagate from the specimen surface to the defect and back, traveling twice the defect depth. This results in severe attenuation and limits the detection depth. To overcome this limitation, this study proposes an opposite-side infrared detection scheme based on high-power thermal excitation with a heat source and an infrared camera placed on opposite sides of the specimen. Experiments were conducted on a composite honeycomb sandwich structure with a layup of 2 mm/28 mm/2 mm, and thermal excitation parameters, including heating power, duration, and acquisition delay, were optimized. The results show that in opposite-side detection, thermal waves travel only one way from the defect to the camera-side surface, enabling clear identification of internal and back-side debonding defects, with a detection sensitivity of 100% for Φ10 mm defects. Therefore, opposite-side transmission infrared detection effectively overcomes the depth attenuation issue caused by round-trip thermal wave propagation in single-sided detection, thereby providing an efficient and reliable method for defect detection in thick-composite honeycomb structures.