Abstract: Compressed air energy storage（CAES） systems always work under off-design conditions due to the factors such as: 1) fluctuation of renewable energy and grid load, 2)variations of ambient temperature and pressure, 3)self-characteristics change of the system components. In this paper, an advanced compressed air energy storage system with thermal storage is studied for its off-design performance.Comprehensive off-design models are established for both components and the entire system. In order to explore the off-design process in depth, the system analysis is divided in to three parts: compression, expansion and the entire system analysis. Moreover, several kinds of exergy efficiency are presented, which could be used to evaluate the compression and expansion sections more properly. The variations of key parameters with time in charge and discharge processes are revealed, including mass flow rate, power, exergy efficiency,hot water temperature, temperatures at exit, pressure/expansion ratio of each stage, and temperature difference/pressure loss of the heat exchangers. It is also found that the system efficiency of sliding-pressure operation mode is 2.08 percents higher than that of constant pressure operation modes.