Objective To compare the effects of preserving the posterior ligament complex (PLC) with torque loading and displacement loading on the biomechanical properties of the adjacent segments after lumbar decompression and fusion. Methods Six fresh male cadaver lumbar specimens of T12-S2 were tested in the sequence of intact, L4-5 laminotomy (preserving the PLC) with fixation and L4-5 laminectomy (destructing the PLC) with fixation, by applying both displacement loading and torque loading, respectively. Ranges-of-motions (ROMs) of the adjacent segments were measured under a noncontact photographic recording system. Results In displacement-loading mode, the flexion ROM in laminectomy group was significantly higher than that in laminotomy group, and no obvious ROM differences were found in the laminectomy and laminotomy groups under extension, lateral bending, and rotation movement. Conclusions Torque loading and displacement loading have different biomechanical effects on the adjacent segments after lumbar decompression and fusion. Compared with the laminotomy method, in which the PLC is preserved, the laminectomy method in which PLC structures are damaged can obviously lead to an increment of flexion ROMs at the adjacent segments, and also increase the ROMs of the adjacent segments under extension, lateral bending, and rotation movements, which might increase the risk of further instability of the fused adjacent segments.