The increase in axle load and speed, the worse of service environment and the deterioration of existing bridges result in various of damages in structures. In order to ensure the safety of the structure, it is essential to assess the performance of the structure. In this paper, based on the frequency changes of the damaged and undamaged structure, damage positioning indicator method was used to locate the damage of the structure. Damage assessment method (SSZD) based on modified modal strain energy method was used to assess the extent of structural damage. The results show that this method can be used to locate and assess the structural damage accurately.