Engine rig tests are performed and compressor passage flow pressure signal at surge boundary is obtained. Spectral analysis is adopted to investigate passage flow characteristics at surge boundary. Among all the frequency contents of the pressure signal of the first compressor stator passage, compared with higher rotor speeds at which the flow matches well, the magnitudes of BPF1 and BPF2 the blades are passing frequency contents for the first two compressor rotors respectively and the magnitudes of harmonic frequency contents near 550 Hz increases obviously at surge boundary, and a few spectral lines with very high magnitudes appear near 550 Hz. In the process of reducing rotor speeds from high speed, the magnitudes of BPF1 and BPF2 rapidly increase at first, and then gradually decrease; while the intensity of the frequency contents near 550 Hz gradually increases with the decrease of rotor speed. As for the test engine, the passage flow characteristics at surge boundary might be helpful to the monitoring and early-warning of compressor operation stability.