Unsteady flows and rotating stall of a low-speed centrifugal compressor are investigated by measuring vaneless diffuser wall static pressure fluctuation and internal flow fields at different small flow fluxes.During the experiment,firstly the real time static pressure fluctuations on the vaneless diffuser shroud at different circumferential and radial position were acquired by high-frequency dynamic pressure transducers.Discrete Fourier transformation analysis and cross-correlation analysis were applied to the experimental results to ascertain the rotating stall beginning operation conditions and stall cells numbers and rotating speed.Secondly,the vaneless diffuser inlet flow angle distribution along diffuser width direction was acquired by single hotwire,which was compared with SENOO’s analysis results.At last,the internal flow fields of the centrifugal compressor were investigated with a particle image velocimetry(PIV)system at different small flow fluxes.The flow field development of vaneless diffuser and blade flow passage are given at rotating stall conditions.The experiments enrich the understanding of rotating stall flow phenomenon of the low-speed centrifugal compressor and provide full experiment data for designing high performance centrifugal compressor. Unsteady flows and rotating stalls of a low-speed centrifugal compressor are investigated by measuring vaneless diffuser wall static pressure fluctuation and internal flow fields at different small flow fluxes. During the experiment, first the real time static pressure fluctuations on the vaneless diffuser shroud at different circumferential and radial position were acquired by high-frequency dynamic pressure transducers. Discrete Fourier transformation analysis and cross-correlation analysis were applied to the experimental results to ascertain the rotating stall initial operation conditions and stall cells numbers and rotating speed. Secondarily, the vaneless diffuser inlet flow angle distribution was along with diffusers width direction was acquired by single hotwire, which was compared with SENOO’s analysis results. At last, the internal flow fields were with particle image velocimetry (PIV) system at different small flow fluxes. The flow field development o f vaneless diffuser and blade flow passages are given at rotating stall conditions. these experiments enrich the understanding of rotating stall flow phenomenon of the low-speed centrifugal compressor and provide full experimental data for designing high performance centrifugal compressor.