Abstract: The CFRP composites of small square tube structure have been applied in new aircrafts. According to the material and structure characteristics of the CFRP square tube, the applicability of the frequently used ultrasonic penetration method, ultrasonic reflection method and entire ultrasonic penetration method to square tube quality inspection is analyzed and compared, and a method of ultrasonic reflection on the opposite inner wall is proposed and used to test the artificial defects in test specimen and actual products. The results show that this method can inspect all the artificial delamination defects of 10mm×10mm in the test specimen and the delamination and hole defects in the actual products.