In order to improve the accuracy of numerical simulation for compressor aerodynamic performance, and to understand the interaction and internal flow characteristics of each compressor component, this paper numerically calculates all mainstream channels coupled impeller backside cavity (IBC) in a centrifugal compressor for compressed air energy storage (CAES). The internal flow field in IBC and the coupling characteristics of the compressor are analyzed under variable operating conditions. Furthermore, the influences of variable rotating speeds of the upstream coupled impeller and variable angles of the downstream coupled adjustable vaned diffusers (AVDs) are studied. The results show that IBC has a unique airflow field structure and vortex flow characteristics. The velocity field, pressure field, temperature field, and other aerodynamic parameters in IBC are different from the mainstream. Due to its coupling relationships with upstream and downstream, variable rotating speeds or AVDs' angles will affect the overall performance of the compressor and internal flow field characteristics in IBC.