Axial-centrifugal combined compressors are prevalent in small and medium-sized aero-engines. However, the understanding of flow instability in combined compressors lags behind that in pure axial compressors. This study aims to investigate the flow instability in a multistage combined compressor using annulus unsteady simulation. The research object is a compressor that consists of three axial stages and one centrifugal stage, designed and tested by Liulingba Company in China. The compressor is used in a recently developed high-speed turboshaft engine. In this study the rotational speed is 24150 rpm, which is 70\% of the design speed. At the impeller inlet tip, the linear speed is 253 m/s, while at the impeller outlet it is 369 m/s. An in-house unsteady RANS solver is employed with a total of 357 million grid cells. The CFD-predicted compressor performance demonstrates a close alignment with the experimental data, displaying a relative error of approximately 3%. At an operational point wherein the entire compressor runs with high efficiency, an unstable flow phenomenon identified as rotating instability is observed in the first axial rotor (R1). The tip vortices of R1 are located almost upstream of the leading edge plane and enter the gap between R1 and S0. Furthermore, a discernible ’hump’ in the frequency spectrum is detected by high-frequency pressure probes set near the leading edge of R1. After filtering the pressure signals, the disturbances arising from rotating instability can be indicated. These distributes rotate at around 50% of the rotor speed, and show intensive unsteadiness. The entire compressor operates stably at this point due to a substantial portion of the pressure increase occurring in the centrifugal impeller.