Objective To explore the biomechanical mechanisms of correlation between lumbar disc degeneration and sacroiliac joint disorder though investigating the biomechanical characteristics of lumbar disc degeneration with sacroiliac joint disorders. Methods One normal healthy volunteer and two patients with lumbar disc herniation and sacroiliac joint disorders (one was with sacrum disorders and the other was ilium disorders) were selected. Their CT data were collected to establish three corresponding lumbar-pelvis finite-element models, and their gait data were also simultaneously collected to drive the AnyBody musculoskeletal model. The muscle force around the lumbar and pelvis as well as the hip joint force were acquired as loading condition for finite element analysis. The stress changes in L4 and L5 intervertebral discs and sacroiliac joints of the patients and normal volunteer were compared. Results There was no significant difference in the stress of the two sides of L4, L5 disc and two sacroiliac joints in normal model, with a bimodal stress curve. However, in the models of sacrum disorders and ilium disorders, the bimodal stress curve peaks changed, even disappeared. The peak stress differences in left and right side of L4 disc were 0.55 MPa and 0.80 MPa, respectively, the peak stress differences in left and right side of L5 disc were 4.05 MPa and 2.08 MPa, respectively, and the peak stress differences in left and right side of sacroiliac joints were 0.96 MPa and 3.32 MPa, respectively. Conclusions The lumbar disc degeneration with sacroiliac joint disorder leads to the tilt of the body loading line, and sacroiliac joint disorder can aggravate the imbalances of stress on the body sides. The impact of sacroiliac joint dysfunction cannot be ignored in the treatment of lumbar disc herniation.