Abstract: There is no mature nondestructive testing method for the internal defects of carbon fiber composite cylinders' periodic inspection at present. X-ray testing technology can undertake qualitative detection for these defects, and CT testing can realize accurate location and quantification of defects. According to the structure characteristics of one type carbon fiber composite gas cylinder for breathing apparatus and relevant periodic inspection standards, various defects were made and the CT testing experiments were conducted. The results show good detection effects for surface defects and fiber layer internal defects. Based on the thickness of aluminum inner container, the location and depth of the defects were estimated.