This paper presents the work on long-term structural health monitoring (SHM) of a 250m high-rise building located in Shanghai, China. The structure resists lateral loads with the steel composite frame and concrete core tube. In 21/F and 36-38/F, ring-shaped trusses and outrigger trusses are used to strengthen the stiffness of the structure. Because the height of this structure is beyond the limit of existing code and its lateral stiffness in the vertical direction is non-uniform, a long-term SHM system is designed to monitor dynamic response of the structure and evaluate the structural condition. The system consists of two parts: acceleration monitoring and displacement monitoring. Based on the system, long-term ambient vibration tests in both construction and using stages were carried out to investigate the modal parameters as well as the lateral displacement of the structure. Operational modal identification methods were employed to analyze the data to determine the dynamic characteristics. The first five modes of the structure and the lateral displacement obtained in different stages are studied and discussed. The results of this paper can be applied to provide reference for the future model updating, damage detection and performance evaluation.