Abstract: Addressing the lack of systematic understanding of the pull-out failure mechanisms of inserts in carbon-carbon honeycomb sandwich structures, this research employed an integrated approach combining pull-out testing, CT scanning, and numerical simulation. Through pull-out testing and CT scanning, the mechanical characteristics and failure modes of carbon-carbon honeycomb sandwich structure during the pull-out process were identified. Utilizing the finite element method based on structural progressive damage analysis, the progression of structural damage was deduced, and the failure mechanism was revealed. Additionally, the influence of insert flange dimension parameters on pull-out failure was investigated, and design optimization guidelines based on the failure mechanisms were proposed. The results demonstrate that the pull-out failure mechanism of inserts in carbon-carbon honeycomb sandwich structures is commonly dominated by face sheet delamination and honeycomb shear failure, exhibiting a distinct two-stage failure characteristic. The initial failure stage is primarily dominated by face sheet delamination, while the final failure stage is dominated by honeycomb shear failure. This mechanism differs fundamentally from the traditional pull-out failure mechanism of honeycomb sandwich structures, which is primarily dominated by honeycomb shear failure. The primary cause of delamination damage in the face sheet is the stress concentration effect due to geometric discontinuity and stiffness variation at the interface between the panel and the insert. The optimized insert with graded flange, designed based on the failure mechanism, achieves a 26% increase in the initial pull-out load and 7.2% weight reduction. The revealed failure mechanisms and optimized design methods for inserts provide theoretical support and engineering guidance for the high-reliability and lightweight design of carbon-carbon honeycomb sandwich structures.