In order to analyze the fatigue life of the posterior thoracolumbar fixed structure, a loading model was established in accordance with the anti-fatigue test requirements specified by ASTM Standard F1717-04. Two three-dimensional Models of the fixed structure with two bars and four bars were built by 3D software (UG), and imported into ANSYS software for static analysis. The maximum and minimum stresses of risk nodes under different loads and moments were obtained. The fatigue life was then calculated using relevant mathematical formula of S-N curve and Goodman curve. It was found that the stress at the middle of the crossbeam between the two bars is larger than the surroundings and is liable to suffer from fatigue.