In this study, a detailed L3-L5, three dimensional nonlinear finite element model of a normal aged 32 man was established with the aid of two softwares, CATIAV5 and MSC. Marc, which can derive a three-dimensional finite element model from CT images. We loaded the model with 10 N * m net moment and computed the average rigidity of functional spinal unit. The model was validated in contrast to the former biomechanical studies of cadaveric spine. The model will be used in further clinical biomechanical studies.