Pedicle screw fixation (PSF) has been extensively utilized in lumbar fusion surgeries to assist in stabilization. However, inappropriate implantation could accelerate the progression of adjacent segment degeneration (ASD). Current finite element studies are predominantly confined to single-segment fixation under conditions of normal bone mineral density and mechanical analyses of unidirectional adjacent vertebrae, failing to fully represent the diversity of clinical conditions. This study aimed to compare the biomechanical effects of long-segment unilateral pedicle screw fixation (UPSF) and bilateral pedicle screw fixation (BPSF) on ASD under normal and osteoporotic bone mineral density conditions. A finite element (FE) comparative study. METHODS Six distinct types of spinal models with L2-L5 fixation were constructed and categorized into two groups according to the different bone mineral density conditions: Group 1 (Model A: bilateral fixation, Model B: left lateral fixation, Model C: right lateral fixation) and Group 2 (Model D: bilateral fixation with osteoporosis, Model E: left lateral fixation with osteoporosis; Model F: right lateral fixation with osteoporosis). Six distinct motions were simulated for each model. These simulations were conducted to analyze the alterations in the range of motion (ROM), intervertebral disc pressure (IDP), articular cartilage stress in the adjacent upper and lower vertebrae, and stress experienced by the screw‒rod system. The ROM of the fused segment decreased significantly after internal fixation, and all values were less than 5°, meeting the stability criteria for surgery. Conversely, the adjacent vertebrae exhibited varying degrees of increased ROM during rotation, with the lower adjacent vertebrae in the osteoporotic model being particularly prominent. In addition, BPSF was more likely to cause IDP and articular cartilage stress than UPSF; the IDP in L1/L2 and L5/S1 was significantly greater in BPSF than in UPSF (difference range: 13.65-16.44%). The stress in the L1/L2 and L5/S1 adjacent vertebral articular cartilages was also greater in BPSF than in UPSF (difference range: 20.19-27.07%). The peak stress of the screw‒rod system was 197.88 MPa, which was significantly lower than its yield stress (825-895 MPa). Both UPSF and BPSF can meet the stability criteria necessary for spinal fusion surgery. The UPSF has relatively little biomechanical influence on adjacent segments, reducing the incidence of ASD. For patients requiring PSF, opting for the UPSF method may be more beneficial, irrespective of the bone mineral density. When treating osteoporosis patients, a critical assessment of bone density is needed, and personalized treatment should be adopted to avoid excessive adjacent vertebral activity and the risk of screw-rod breakage. This study provides strong biomechanical support for the clinical selection of PSF types for patients with different bone mineral density conditions.