Objective To study the biomechanical influence of posterior laminectomy with varying extent on adjacent segment after lumbar interbody fusion. Methods Three finite element models of lumbar posterior fusion were developed based on the validated intact lumbar model. These models were: posterior fusion with bi-lateral incision of facet joint (Bi-TLIF），inferior partly incision of laminar (PLIF)，total laminectomy (LAM-PLIF). The range of motion (ROM), intradiscal pressure (IDP), facet joint contact force (FJF) of adjacent segment of fusion models under various loading were compared with the intact model. The follower load of 400 N under 7.5 N·m torque was exerted on superior endplate of L1 segment. The 6-DOF (degree of freedom) of sacroiliac joint surface was constrained during loading. ResultsDuring flexion, obvious biomechanical changes of superior adjacent segment (L3-4) were found in Bi-TLIF, PLIF, LAM-PLIF surgery groups. Compared with the intact model, the ROM in Bi-TLIF, PLIF, LAM-PLIF group increased by 1.0%, 9.3%, 24.5%, respectively, while IDP in the above fusion groups increased by 1.4%, 4.3%, 10.0%，respectively. These changes were not obvious in other postures. For FJF, the Bi-TLIF and PLIF group showed obvious increasing effect on L3-4 segment, while almost had no effect on L5-S1 segment. Conclusions Laminectomy increased ROM, IDP and FJF of adjacent segment (especially superior adjacent segment) after posterior lumbar fusion, which might increase the risk of adjacent segment degeneration. This biomechanical effect was more obvious with the increase in incision range of laminar. Therefore, preserving more posterior complex during decompression has a positive effect on preventing adjacent segment degeneration (ASD) following lumbar fusion surgeries.