As demonstrated by former work,the holed casing treatment can be used to expand the stall margin of a centrifugal compressor with unshrouded impeller.In addition,the choked margin can also be expanded as experimental results indicated.Moreover,the compressor performance,especially the efficiency,on the whole working range is improved.As shown by experiments,the stall margin and choked margin of the compressor are extended,and the maximum efficiency improvement is 14%at the large flowrate of 1.386.Numerical simulations were carried out to analyze the flow in the impeller and in the holes in the case of large flowrate.The results indicate that in large flowrate conditions,there is a low-pressure region on the throat part of the impeller passage,leading to the bypass flows appearing in the holes,which means the flow area at the inlet of the impeller is increased.The bypass flow can also contribute to the decrease of the Mach number at the throat part near the shroud end-wall which implies that the choked margin is expanded.Besides,as the bypass flow would inhibit the development of the vertexes in the tip clearance and suppress the flow recirculation in the shroud end-wall region,both the pressure ratio and efficiency of the compressor are improved,which agrees well with the experiments. As demonstrated by former work, the holed casing treatment can be used to expand the stall margin of a centrifugal compressor with unshrouded impeller. Addition, the choked margin can also be expanded as experimental results indicated. , on the whole working range is improved. As shown by experiments, the stall margin and choked margin of the compressor are extended, and the maximum efficiency improvement is 14% at the large flow rate of 1.386.Numerical simulations were carried out to analyze the flow in the impeller and in the holes in the case of large flowrate. The results indicate that in large flowrate conditions, there is a low-pressure region on the throat part of the impeller passage, leading to the bypass flows appearing in the holes, which means the flow area at the inlet of the impeller is increased. The bypass flow can also contribute to the decrease of the Mach number at the throat part near the shroud end-wall which implies that the choked margin is expanded.Besides, as the bypass flow would inhibit the development of the vertexes in the tip clearance and suppress the flow recirculation in the shroud end-wall region, both the pressure ratio and efficiency of the compressor are improved, which agrees well with the experiments.