In order to realize the detection of internal defect in composite insulator, we presented a non-destructive testing method based on microwave, which utilizes the micro wave's refraction and reflection features on the interfaces of different dielectrics. Based on electromagnetic field theory, analysis of the reflected signal amplitude under different conditions were made, which proves the feasibility of microwave reflection method for composite insulator internal defect detection theoretically. Then, a microwave detection platform for composite insulator was built in this paper. With the aid of this platform, a test insulator which is collected from operation site for the abnormal temperature was detected and also anatomized for the validation of test result. The results show that distinguishing reflected signal to different defects were obtained with this new method and the feasibility of microwave reflection method for composite insulator internal defect detection was verified.