In this study, a non-invasive and environment-friendly acoustic emission (AE) technique, coupled with multiple analysis methods, was proposed to monitor the critical variation of separation efficiency of a cyclone separator by means of the gas-solid hydrodynamics. Acoustic energy analysis was used to reflect the characteristics of particle motion and particle-wall interaction. Shannon entropy analysis was adopted to reflect chaos and complexity of gas-solid two-phase motion. Multi-scale analysis (Hurst analysis and wavelet analysis) was used to obtain multi-scale characteristics inside the cyclone separator. To be specific, micro-scale collision behaviors of particle-particle and particle-wall, meso-scale interaction behavior between particle clusters and gas phase, and macro-scale interaction behavior (overall flow behavior). Furthermore, by comparison with the results of separation efficiency and pressure drop, the above-mentioned three methods based on AE technique can effectively reflect the critical variation of separation efficiency for cyclone separator.