Objective To establish the fatigue test device and observe the stiffness of two spinal posterior fixation devices in the fatigue test. Methods The testing apparatus consisted of two round plate which fixed specimens. The pivot of upper plate, which was fixed balance rods, was made of a 3 cm anterior offset to the specimens. To simulate the effect of erector spinae muscle force on the spinal segment, a sping was mounted between the two rods posterior the specimens. In ten specimens using lumbosacral cadaveric spine(L1-S2), with an L3 burst fracture, two dorsally applied spinal fixation implants(Reduction Fixtion,RF and Modified Edwards System,MES)were used acording to clinical operation. In SANS testing machine, the specimens were loaded cyclically at 0.5Hz in(440±165)N for up to 10,000 cycles. Displacement transducers monitored motion at the site of instability and at the segment below the implants. Flexibility at these segments were measured at initial and 1500 and 10000 cycles. Results In the fracture models, a one-way analysis of variance showed that there were more significant differences in flexibility of the two fixation constructs(P<0.05). The stiffness of RF and MES was increased at initial cycles. RF allowed significantly more rigid across the site of instability than MES at 1500 and 10000 cycles. There were no significantly more stiffness between the fixation constructs and intact spine at the segment below the implants. Conclusion This fatigue test device is fit for biomechanical study of spinal interal instrumentation.