The characteristics of nonlinear seismic behavior and failure mechanism of RC space frame in railroad viaducts have been studied by the numerical analysis in time domain. The structure concerned is modeled in 3 dimensional extent and the RC frame elements consisting of fibers are employed for the columns. The fibers are characterized as RC zone and PC one to distinguish the different energy release after cracking resulted from the bond characteristic between concrete and re-bar. Due to the deviation of the mass center and the stiffness center of the entire structure the complex behavior is shown under seismic actions. The excessive shear force is concentrated on the column beside flexible one relatively, which leads to the failure of bridge concerned.