Due to the characteristics of the process, 3D-printed composite materials will have additional pores compared to those manufactured by traditional methods. Therefore, it is of crucial significance to study the distribution of pores, the methods for reducing them, and the influence of pores on the mechanical properties of materials. In this study, the composite materials were imaged with X-ray computed tomography in 3D, and semantic segmentation of pores and cracks was carried out based on U-Net, based on which detailed analysis of the defects has been accomplished. It is found that there are more smaller pores in the −45° and 45° plies, presumably because the nozzle decomposed the oblique displacement into multiple 0° and 90° micro-displacements during the printing process. Meanwhile, pores with a volume ranging from 103 to 106 μm3 predominantly exist within each lamina whereas pores with larger or smaller volumes mainly exist in the interlaminar regions. Additionally, compression tests show that post-processing by compression moulding shifts the dominating damage mode from kink band to delamination under compression.