Abstract: The low-density thermal protection and insulation material is an important constituent material on aerospace craft thermal protection system. The material is low density, high porosity, and low thermal conductivity, with lightweight and excellent thermal protection and insulation performance. However, the material and its microscopic structure characteristics cause difficulties on non-destructive testing on the internal defects. The regular ultrasonic transmission testing is unsuitable, and the infrared testing is less effective. To detect the delamination defect of low-density thermal protection and insulation material, which is composed of quartz needled fabric and phenolic resin, the research on air-coupled ultrasonic testing is carried out. The microstructure of the material was analyzed by X-ray micro-CT detection. The relationship between delamination defect sound pressure transmittance and air gap thickness was estimated. The material specimens with densities of 0.4, 0.5, 0.6, 0.7 g/cm³ were made. The research of air-coupled ultrasonic testing on the specimens with probe frequencies of 50, 140, 200 kHz was performed. The result shows, the air-coupled ultrasonic testing is effective on delamination defect detection of low-density thermal protection and insulation material. The suitable detection frequency and detectability are both related to the material density and the material homogeneity. When the material thickness is 30 mm, the delamination defect air gap thickness is 0.3 mm, the probe frequency is 50 kHz, and the material density is within 0.4~0.7 g/cm³, the delamination defect with diameter greater than 30 mm can be detected.