The insulation system is an important part of the fusion magnet coils and its role is to mechanically support, insulate, fix, and protect superconducting magnets. The insulation materials used for the China Fusion Engineering Test Reactor (CFETR) Central Solenoid Model Coil (CSMC) are glass-fiber reinforced plastics. Internal defects may be generated in the composite insulation structure during manufacture or service life. The inspection of the composites has many difficulties, mainly due to the multiple interfaces and different elastic properties. This article investigated the microstructure of the insulation system and defined the test objects for non-destructive test(NDE). Phased array ultrasonic test (PAUT) technology was developed to evaluate the integrity of the insulation system.