Objective: To evaluate the biomechanics of the bilateral pedicle screw (BPS) and the bilateral modified cortical bone trajectory screw (BMCS) fixation technique in the posterior lumbar interbody fusion (PLIF) model at the L4-L5 segment. Methods: Finite element models of the L1-S1 lumbar spine are established based on three cadaveric lumbar spine specimens. BPS-BPS (L4-L5, TT), BPS-BMCS (L4-TT and L5-MCBT), BMCS-BPS (L4-MCBT and L5-TT), and BMCS-BMCS (L4-L5, MCBT) are implanted into each FE model. Range of motion (ROM) at the L4-L5 segment, and von Mises stress on the internal fixation system, Cage on connecting rods are compared under bending, extension, flexion, and rotation conditions with 400N load and 7.5 Nm torque. Results: BMCS-BPS showed lower ROM, and von Mises stress on the Cage, internal fixation system, and connecting rods under rotational conditions compared to other groups. BPS-BMCS and BMCS-BPS significantly reduce the ROM of the L4-L5 segment under bending and rotational conditions compared to BPS-BPS and significantly decrease ROM under rotational conditions compared to BMCS-BMCS. BPS-BMCS and BMCS-BPS have a significantly reduced risk of Cage subsidence under bending conditions compared to BPS-BPS and rotational conditions compared to BMCS-BMCS. BPS-BMCS and BMCS-BPS significantly reduce the risk of connecting rod fracture under bending and rotational conditions compared to BPS-BPS, and BMCS-BMCS, enhancing the stability of the internal fixation system. Conclusion: PLIF combined with BPS-BMCS and BMCS-BPS fixation technique can offer better stability of the internal fixation system, vertebral stability, and lower risk of Cage subsidence and connecting rod fracture during bending and rotation in the human body, thereby improving surgical success rate and patient recovery outcomes.