The sacroiliac joint (SIJ) constitutes the predominant pain source following lumbar or lumbosacral fusion. Although studies have investigated the biomechanical patterns of SIJ behaviours 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, sacroiliac joint and 7 mainly ligaments. After successful validation, the model was used to investigate the biomechanics of sacroiliac joint and ligaments in simulating lumbosacral fusion process. Our results showed that small motion in a stable SI joint may significantly increases the contact pressure and stress of the sacroiliac joint, 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 sacroiliac joint possibly causes pain at the sacroiliac joint, 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 work may contribute to understand the relationship between sacroiliac joint ligaments and sacroiliac joint pain. This article is protected by copyright. All rights reserved.