Objective To measure and compare the stresses along the screws in three-segment and two-segment posterior internal fixation in spondylolishesis.Method Six fresh samples of human lumbar vertebras were used in the study.The spondylolishesis model was made by Panjabi method,and was placed with the universal spine system of SINO.By strain measurement and biomechanical measurement,the samples were tested in 5 different directions with INSTRON 5567.Stress along the screws for two-segment(group A) and three-segment(group B) were measured respectively.Result The draw-screw in group A shared more stress than that in group B(P<0.05);The fixed-screw in group B shared more stress than in group A(P<0.05)?.Conclusion The three-segment internal fixation can effectively decrease the stress along the draw-screw,and depress the probability of breakage of the screw due to the system fatigue.