Abstract: Due to the effect of damping,the mode shapes for bridges identified by an instructed vehicle or contact responses are often distorted.Thus,a PSO-HT algorithm based on the combination of Particle swarm optimization (PSO) and Hilbert transform (HT) was proposed.The algorithm can realize the synchronous automatic identification of bridge frequency,damping ratio and mode shape.Also,the interference of damping to the identification of mode shapes was eliminated.Firstly,the closed-form solution of a moving vehicle passing through a damped simply supported bridge was derived.Secondly,the theoretical background for the PSO-HT algorithm was proposed by analyzing the frequency components of vehicle and contact point response and combining with PSO and HT methods,and the specific implementation steps of the algorithm were given.Then,the correctness of the theoretical derivation and PSO-HT algorithm was numerically verified.Finally,the road surface roughness,vehicle speed and bridge damping were selected as the core influencing factors to verify the reliability and robustness of the PSO-HT algorithm.The results show that the closed-form solution is in good agreement with the numerical solution,which confirms the reliability of the theoretical derivation.The proposed PSO-HT algorithm can extract the first few frequencies and damping ratios accurately.The influence of damping ratio on the mode shapes can be well eliminated.Although the pavement roughness has a certain influence on the high-order mode shape recognition,it has little influence on the low-order ones.Relatively,the vehicle speed and bridge damping have limited influence on the PSO-HT algorithm,which shows that the algorithm has strong robustness.The research results provide a new idea for vehicle scanning method to identify the bridge mode shapes.