Abstract: In order to solve the problem of nondestructive tesing of glass fiber reinforced plastics (GFRP) composites for superconducting magnet insulation, this paper studies the acoustic properties of GFRP and carries out ultrasonic testing experiments. Ultrasonic detection of laminates with thickness of 1. 21-4. 39 mm and insulating layer test blocks with actual defects are carried out. Combined with metallographic observation, the defect detection results of insulation layer test blocks are analysed, and the influence of material multiphase heterogeneity on defect detection is discussed. The test result shows that the sound velocity of the laminate sample is between 2 698. 75-3 256. 41m ·s^-1, and the time domain attenuation coefficient is between 5. 03-12. 58 dB ·mm^-1, and the interlayer reflection can be observed in the ultrasonic signal. For 1. 21 and 3. 32 mm thick laminates, artificial defects of ϕ 5. 0, ϕ 3. 0 and ϕ 1. 0 mm can be detected, for 4. 39 mm thick laminate, only artificial defects of ϕ 5. 0 and ϕ 3. 0 mm can be detected,which provide some reference for ultrasonic detection of GFRP materials.