Abstract: A new method for determining the axial position of treated casing in multistage axial flow compressors is set up in this paper.Firstly,the stalling pressure rise capability of axial flow compressor stages is predicted,and the stage in which stall occurs first could be located theoretically and be verified experimentally.Secondly,the stage stall appearing first is taken as the axial position of treated casing.The idea is verified in a systematic way with the help of four kinds of different casing treatment in a subsonic two stage axial flow compressor.Experimental results indicate that the treated casing placed on the first stalling stage could improve the stall margin better.The method presented in the paper could be used as the guide in the casing treatment design for determining the axial position of the treated casing.