Objective To investigate the biomechanics changes of the reconstructed pelvis after sacrectomy. Methods Three three-dimensional finite element pelvic models were established: (1)a normal pelvic model, (2)a defective pelvic model on which sacrectomy was operated between S1 and S2 sacral vertebrae, and(3)a reconstructed pelvic model. Sitting posture was stimulated under the load of body weight, and the stress and displacement distributions of these models were calculated and compared. Results The displacement of anchor point on the sacral promontory, the stress distributions of the pelvis and the reconstruction instruments of the three models were all obtained. Conclusion Sacroiliac joint reconstruction can increase the stiffness of the defective pelvic and decrease the maximum stress around the sacroiliac joint, thus reducing the breakdown risk of the defective pelvis. Part of titanium sticks between L5 pedicle screw and iliac screw is the weakest region in the instrument. It is suggested that the bending of titanium sticks should be conducted carefully without forming acute angle and significant stress concentration on the titanium sticks so as to reduce the risk of sticks breakdown.