Abstract: Six types of three-dimensional (3D) angle-interlock woven preforms of quartz fiber with different warp densities were prepared using the 3D weaving process. A 3D profilometer was employed to characterize the mesoscopic structure of the internal cross-section of the preforms. Three quantitative indicators, namely yarn crimp rate, cross-sectional flatness ratio, and cross-sectional area, were proposed to characterize the geometric deformation of the yarns. The changes in the mesoscopic structure of the yarns within the preforms under different warp arrangement densities were compared. Additionally, a ridge regression method was used to establish the relationship between the yarn geometric deformation in the preforms and the fiber volume fraction, thereby revealing the influence mechanism of warp density on the fiber volume fraction of 3D angle-interlock woven preforms under the weaving limit state.The results show that compared with the yarn crimp rate and cross-sectional area, the cross-sectional flatness ratio has a more significant impact on the fiber volume fraction and is the main influencing factor. The weft flatness ratio is the key to increasing the fiber volume fraction. As the warp density increases, the wefts are compressed, leading to an increase in their flatness ratio (the weft flatness ratio reaches 24.9% when the warp density is 10 yarns/cm). This effectively fills the gaps between the warp and weft yarns, resulting in a maximum fiber volume fraction of 49.1%.