Objective To evaluate the biomechanical properties of the bilateral pedicle screw (BPS) and bilateral modified cortical bone trajectory screw (BMCS) fixation techniques in the posterior lumbar interbody fusion (PLIF) model of the L4–5 segment. Methods Finite element models of the L1–S1 lumbar spine were established using three cadaveric lumbar spine specimens. BPS-BPS (TT at the L4–5 segment), BPS-BMCS (TT at the L4 segment and MCBT at the L5 segment), BMCS-BPS (MCBT at the L4 segment and TT at the L5 segment), and BMCS-BMCS (MCBT at the L4–5 segment) were implanted into the finite element model. The range of motion (ROM) at the L4–5 segment and the peak von Mises stress on the internal fixation system, cage, and connecting rods were compared under bending, extension, flexion, and rotation conditions with a 400 N load and 7.5 N·m torque. Results The BMCS-BPS group showed lower ROM and von Mises stress on the cage, internal fixation system, and connecting rods under rotational conditions than the BPS-BPS, BPS-BMCS, and BMCS-BMCS groups. The BPS-BMCS and BMCS-BPS groups had significantly reduced ROM of the L4–5 segment under bending and rotational conditions compared with the BPS-BPS group, and significantly decreased ROM under rotational conditions compared with the BMCS-BMCS group. The BPS-BMCS and BMCS-BPS groups had a significantly reduced risk of cage subsidence under bending conditions compared with the BPS-BPS group, and under rotational conditions compared with the BMCS-BMCS group. The BPS-BMCS and BMCS-BPS groups had a significantly reduced risk of connecting rod fractures under bending and rotational conditions compared with the BPS-BPS and BMCS-BMCS groups. This enhanced the stability of the internal fixation system. Conclusions PLIF combined with the BPS-BMCS and BMCS-BPS fixation techniques can provide better stability for the internal fixation system and vertebral body as well as a lower risk of cage subsidence and connecting rod fracture during bending and rotation in the human body. This would improve the success rate of surgery and recovery effect of patients.