To clarify the impact of combined defects of lining voids and insufficient thickness on tunnel structures, a three-dimensional finite element numerical model is established to simulate the combined defects of lining insufficient thickness and behind-lining voids. The study investigates the variation patterns of structural deformation, internal forces, and safety performance under different parameters (defect size and location). The results indicate the following: (1) The combined defects of lining voids and insufficient thickness reconstruct the surrounding rock-lining stress field, causing the originally compressed side at the crown to transition from compressive stress to tensile stress, while stress concentration at defect edges intensifies. When the defect size increases to a critical value (longitudinal defect length ≥ 6 m or insufficient thickness ratio ≥ 2/5), the lining deformation increases by over 200% compared to the defect-free condition. Moreover, multi-defect coupling (crown + bilateral haunches) induces asymmetric deformation, with displacement variations reaching 53%. (2) The combined defects trigger internal force redistribution, increasing the extreme bending moment to 3.1 times that of the defect-free condition, while axial force decreases to 38%－53%. Under multi-defect coupling conditions, the bending moment at the spandrel increases by 104%, whereas the increase under single-defect conditions is only 50%－70%. (3) The combined defects significantly reduce the sectional safety factor. When the defect size reaches a critical value (void range > 34° or insufficient thickness ratio ≥ 2/5), the circumferential and longitudinal safety factors of the lining section decrease by 40%－60% and 60%－70%, respectively. In contrast, the reduction under singledefect conditions is typically less than 40%, and the critical value is notably lower than that for single defects.