The compressor's aerodynamic instability problem of a certain turbojet engine in the process of the throttling operation was experimentally studied. It is observed that different auto-correlation functions of the pulsating components of the casing wall static pressure in the first compressor stage correspond to the different working conditions of the aero-engine. An eigenvalue “ μ”is then introduced to quantify the variation of auto-correlation functions. The value “ μ”is capable of reflecting some aspects of flow separation in the tip region of the first compressor stage, and may serve as an indicator of imminent aerodynamic instability of the compressor. Besides, it is promising to apply the auto-correlation algorithm to real time control.