BACKGROUND: Spinopelvic sagittal parameters have a significant influence on adjacent segment degeneration (ASD) after fusion surgery. The association between ASD and sagittal balance is not well understood. The purpose of this study was to investigate the biomechanical influence of various sacral slope (SS) degrees on adjacent segments after transforaminal lumbar interbody fusion (TLIF) at the L4-L5 level. METHODS: We conducted a finite element model of the L1-S1 based on computed tomography scan images. The L1-S1 model with L4-L5 TLIF was modified with various SS degrees (33°, 38°, 43°, and 48°) to investigate the biomechanical influence of SS on adjacent segments. The range of motion (ROM) and intradiscal pressure (IDP) of the adjacent segments (L3-L4 and L5-S1) were compared among models using various SS angles. RESULTS: When the SS angle increased, the ROM and IDP in L5-S1 decreased gradually after TLIF at the L4-L5 level in all motion patterns. Nevertheless, the ROM and IDP in L3-L4 were not significantly different among various SS angles. CONCLUSIONS: Decreased SS after lumbar fusion surgery may pose a higher risk of ASD. Therefore, restoring appropriate SS should be considered during decision-making prior to fusion surgery to reduce the risk of degenerative changes.