An approach for identification of the aero-engine surge precursors was proposed utilizing microphone array measurement, thereby the additional disturbance of classical intrusive measurement was avoided. A transient test on a multi-stage fan experiment was carried out from designed operating condition to surge margin, and the surge inception was realized by signal processing approaches including short-time Fourier transform, spectral analysis and acoustic mode analysis. The results demonstrated the occurrence of asynchronous frequency when the fan operated near the surge margin, and the frequency was about 70% of the blade passing frequency. The cause of such phenomenon was the resonance of azimuthal modes inside the rotor-stator row. The surge-warning was realized by characterizing the feature frequency and mode spectrum.