In recent years, terahertz technology as a new detection technology has developed rapidly, but the detection of carbon fiber composites has great limitations. In this paper, firstly, the electrical anisotropy and skin depth of carbon fiber composites are analyzed, and then the experimental samples are tested by THz time-domain spectroscopy. Finally, the results of calculation and experiment are compared and analyzed. The results show that in the transmission mode, the detection ability is related to the skin depth, which is further verified by theoretical calculation. In this paper, a quantitative explanation for the limitation of THz detection of carbon fiber composites is given for the first time, and a new content for THz nondestructive testing of carbon fiber composites is added.