Objective: To present the ligament effects on sacroiliac joint (SIJ) stability and human pelvis biomechanical characteristics in two different positions by using three-dimensional finite element models (FEM) of pelvis. Methods: Based on the CT data of human pelvis, three-dimensional FEMs of human pelvis in sitting and standing positions were established, which include the bone (sacrum, ilium and coccyx) and six ligaments (sacroiliac, sacrospinous, sacrotuberous, inguinal, superior pubic and arcuate pubic ligaments). 600N vertical load was applied at the upper surface of sacrum to analyze the stress and displacement distribution of pelvis and SIJ. Results: The simulation results demonstrated that the maximum stresses of sacrum and ilium on SIJ contact surface were 5.63MPa and 7.40MPa in standing position and 7.44MPa and 7.95MPa in sitting position. The stresses of ligament dysfunction group were higher than that of health group, which increased by 22.6% and 35.7% in standing position and 25.2% and 43.6% in sitting position in sacrum and ilium. The maximum displacements located on the upper surface of sacrum, which were 0.13mm and 1.04mm in standing and sitting positions, Ligaments dysfunction group increased 30.7% and 9.6% than health group in standing and sitting positions. Conclusions: The location of stress concentration and displacement distribution of pelvic bone are closely resembled previous research results in two different positions. The simulation results may provide beneficial information and theoretical models for clinical research of pelvic fracture, joint movement and ligament functional injuries.