Objective The thoracolumbar vertebra fracture is usually treated by transpedicle screw fixation. Since its effects are correlated to such factors as the length and the implanted direction of screw ,BMD etc. This study was made to provide some biomechanical evidence for better clinical operation.Methods To take six cadaveric lumbar spines to undertake tests with experimental stress analysis.Results The strain of long screw compared with short ones,is respectively 16%, 41%, 56%, 41% lesser in compression,anterior flection,posterior extension and luteral flection;the strength is improved by 16%, 41%, 55% and 41%; the displecement of spine is lower by 18%, 25%, 32% and 30%;the axial rigidity is improved by 18%, 25%, 32% and 30%(P<0.05).The drawing strength of screw implanted in parallel direction is larger by 20% than screw implanted in 7° direction,while the drawing displacement increased by 7%,the energy increased by 18%(P<0.05). With regard to the bone density effect on drawing strength, the normal group is larger than osteoprotic group by 67%. There is average 16% difference in the relative displacement ,strain and energy.Conclusion The relative factors of the fixation that mentioned above should be caused attention in operation so that it can improve the operative quality and reduce complications.