Structural modal parameter identification is generally the foundation and emphasis of structural health monitoring. It can evaluate the structural state and identify structural damage, which is of great significance to the subsequent maintenance of the structure, as well as the ancient building structure. Xi'an city wall belongs to the first batch of key cultural relic protection units in China, and its modal parameter identification needs more attention. Based on the monitoring system of velocity and acceleration dynamic characteristics of the wall at the Wengcheng of Xi'an urban wall, the vibration response data of the urban wall under the excitation of complex traffic environment were obtained through the field dynamic test. The structural modal parameters of the response data were identified by the combination of the three modal identification methods of the feature system realization algorithm (ERA), the random subspace method (SSI) and the peak value method (PP) to obtain the modal parameters of the urban wall in the region. Combined with the numerical finite element model, the modal parameters obtained by the three methods were compared to establish an effective structural dynamic model. The results show that these three methods can effectively identify the modal frequency of the structure under environmental excitation and finally obtain the vibration mode, which verifies the feasibility of the three modal identification methods for the modal parameter identification of the ancient city wall.