<p>To explore the characteristics of stall precursors and the flow mechanism of an axial-flow compressor, this paper takes the single-stage axial-flow compressor of Northwestern Polytechnical University as the research object. Based on the Riemann Invariant boundary conditions, a six-channel numerical simulation method is adopted to investigate the stall precursors of the compressor and the flow-field details before and after the occurrence of stall. During the calculation process near the stall point, different from the previous throttle-valve models that continuously modify the boundary conditions, this paper uses the Riemann Invariant boundary conditions, which can calculate the near-stall point more quickly and conveniently. The monitoring shows that the static-pressure change during the stall process can be divided into three stages: stable, random fluctuation, and periodic oscillation. At the same time, by analyzing the Mach number in the passage and the unsteady flow at the blade tip, the mechanism of stall caused by the movement and break-up of the leakage vortex is revealed. Finally, according to the phase difference of the static-pressure change curves in multiple channels, it can be obtained that the circumferential movement speed of the stall cell in the passage is approximately 62.7% of the rotor rotation speed. Compared with the experimental value of 67.08%, the error is 6.53%. This accurately predicts the movement speed of the stall cell in the passage and further verifies the accuracy of studying the characteristics of compressor stall precursors based on the Riemann invariant boundary conditions.</p>