OBJECTIVE To explore the effects of posterolateral lumbar fusion (PLF) and posterior lumbar interbody fusion (PLIF) on the stability of postoperative unstable single lumbar segment and the biomechanical alterations of the adjacent segments. METHODS A finite element model of L3-S1 segments with a single segmental degeneration at the L4-5 level was established, and the model of L4-5 segmental instability after posterior laminectomy and facetectomy was also established, in which laminar and interior 2/3 area of bilateral facet joints were resected. Physical loads were applied to the models and the changes of the range of motion (ROM) at L4-5 level in different models were recorded at the condition of flexion, extension, lateral bending and rotation. PLF and PLIF were performed on postoperative unstable model respectively, the changes of the ROM at L4-5 level, the ROM and the stress on the adjacent discs in different models were recorded. RESULTS Compared to the unstable model, the L4-5 segmental stability was restored after PLF or PLIF. The ROMs of L3-4 and L5-S1 levels were similar to the preoperative unstable model. The stress on adjacent discs (L3-4 and L5-S1) was increased significantly, and maximum stress distribution changed and concentrated in the anterior annulus fiber in the two fusion models. There was no significant difference of the maximum stress on adjacent discs between PLF and PLIF models [(1.056 ± 0.061) mPa vs (1.070 ± 0.075) mPa; (1.147 ± 0.055) mPa vs (1.162 ± 0.075)mPa, P>0.05]. CONCLUSIONS Lumbar segmental stability after posterior laminectomy and facetectomy can be recovered by both PLF and PLIF. Both PLF and PLIF may increase the possibility of adjacent segment degeneration because of the augmentation of maximum stress on adjacent discs.