Objective To explore the impact of different lamina formation ranges on the biomechanical stability of L5, S1 in spine surgery with unilateral biportal endoscopy (UBE), providing a theoretical basis for optimizing clinical surgical plans. Methods A complete lumbar finite element model (M0) was constructed based on CT data of L3-S1 from a healthy male volunteer. Four different UBE surgical models with varying lamina formation ranges (M1-M4) were simulated. M1 model involved initial laminectomy with essentially intact facets; M2 model involved minor facet resection (5-10 mm from the inferior facet joint surface); M3 model involved greater facet resection with partial laminectomy depth >10 mm; M4 model involved complete facet resection to simulate extreme decompression. Finite element analysis was performed to assess the range of motion (ROM), maximum displacement, and maximum von Mises stress of the vertebrae under different physiological activities (flexion, extension, left/right bending, and left/right rotation), as well as the maximum displacement and maximum von Mises stress of the intervertebral disc, and the maximum von Mises stress of right facet joints under left rotation and right bending. Results With increasing forming range, the ROM of the vertebrae in flexion showed a slight increase (0.32° higher in M4 model than in M0 model), and the maximum displacement generally increased in all motion states. For the intervertebral disc, the maximum von Mises stress and displacement increased mildly in flexion and left rotation, which were approximately 17% and 12% higher in M3 and M4 models than in M0 model, respectively. And the biomechanical parameters changed little among different models under extension, right rotation, and left bending. The von Mises stress of the right facet joint increased stepwise with forming range during left rotation (about 57% higher in M3 model than in M0 model) and was higher in all surgical models than in M0 model during right bending. ConclusionExpanding the lamina formation range in UBE spine surgery can lead to reduced stability in flexion and left rotation activities at L5, S1, increasing the mechanical load on the intervertebral disc and facet joints. Clinically, under the premise of achieving adequate decompression, prioritizing a forming range corresponding to the lower transverse width partition (25%-50%) may better balance decompression efficacy with biomechanical stability of the L5, S1 segment, thereby reducing the potential risk of long-term degeneration caused by excessive bony resection.