The sacroiliac joint (SIJ) constitutes the predominant pain source following lumbar or lumbosacral fusion. Although studies have investigated the biomechanical patterns of SIJ behaviors after lumbosacral fusion, the relationship between ligament strain and SIJ pain following lumbosacral fusion remains unclear. The present study developed a three-dimensional finite element model including L4, L5, sacrum, ilium, SIJ, and seven mainly ligaments. After successful validation, the model was used to investigate the biomechanics of SIJ and ligaments in simulating lumbosacral fusion process. Our results showed that small motion in a stable SIJ may significantly increases the contact pressure and stress of the SIJ, which increase the maximum contact pressure by 171%, 676%, 199%, and 203% and stress by 130%, 424%, 168%, and 241% for flexion, extension, bending, and axial rotation, respectively. An increase in contact pressure and stress in SIJ possibly causes pain at the SIJ, especially in extension and axial rotation. A comparison between the lumbosacral and intact models exhibited the maximum strain increase in the iliosacral ligament (ISL) and the ileal ligament (IL) under all loading conditions. The present study suggests that after lumbosacral fusion process, the ligament sudden increase or decrease is likely to lead sprain or strain ligament, especially ISL and IL thereby causing SIJ pain. This study may contribute to understand the relationship between SIJ ligaments and SIJ pain.