Compressed air energy storage (CAES) system is an "electricity to electricity" device. To reveal the energy conversion process and understand the energy loss principle are critical to improve the energy conversion efficiency. In this paper, the charge/discharge process analysis of an axial turbine based CAES with constant volume is executed in constant turbine inlet pressure mode and variable turbine inlet pressure mode. Firstly, the energy efficiency analysis of a whole round trip cycle of CAES system under design condition is carried out. The second law efficiencies of the operation mode 1 and the operation mode 2 are 44.56% and 45.16%, respectively. It also can be found that the throttling loss in operation mode 1 can be offset by the benefits of higher isentropic efficiency of turbines. Then, the off-design analysis of these two operation modes is implemented by considering the different power levels and speed levels. The mass flow rates of working fluids across the expansion train both increase with the increased load rate or the decreased speed. The load rate or speed both have the positive effect on the exergy efficiency and the energy generated per unit volume of storage, but have the negative effect on the heat rate. (c) 2015 Elsevier Ltd. All rights reserved.