In order to evaluate the damage of steel structures induced by earthquake, this research investigated the coupling relation between the ultimate plastic deformation and low-cycle fatigue life as well as the cyclic softening behavior of structural steel. The repeated high strain loading experiments were carried out on low carbon steel Q345qC, and the low-cycle fatigue life and cyclic softening behavior were studied. By analyzing the relationships between the bearing capacity deterioration and the dissipating energy density, the modification of the bounding surface radius and center and cyclic softening were introduced to the two-surface constitutive model. On this basis, the mechanical behaviors of a steel bridge pier under cyclic loading were investigated by employing the modified two-surface constitutive model, and the effects of material cyclic softening on the bearing capacity of the steel bridge pier were analyzed and discussed. Results show that the structural steel Q345 has good ductility and anti-low-cycle fatigue property. No evident coupling relation between ultimate plastic deformation and low cycle fatigue life of the steel was observed in the experiments. Cyclic softening is more apparent at a larger plastic strain state. The modified two-surface constitutive model is capable of simulating the cyclic softening behavior of the steel in a very satisfactory manner. The bearing capacity of the steel bridge pier under cyclic loading or in earthquake decreases slightly when material cyclic softening is considered.