Abstract: The 3D meso-structures of the fiber preforms was reconstructed based on the micro-computed tomography (micro-CT) technology, two quantitative indexes were proposed to characterize the deformation of the fiber preforms' geometric structure. The influence mechanism of weft densities and thicknesses on the meso-structure of the 3D woven preform was studied. The results show that the micro-CT technology can be used to characterize the cross-section and spatial path of the yarns inside the 3D woven preform effectively. The preform samples with the weft density of 2.0 picks/cm show obvious deformation, and thus are not suitable for practical engineering application. With the increase of the weft density, the internal structure of the fiber preform tends to be stable. The internal structures of the 10 mm samples are more stable than that of the 5 mm samples, however, the cross section and path of the surface yarns still show large deformation.