“Two-regime-surge” is a special instability behavior of compressors, which was investigated in this paper. When the compressor operates at medium rotor speed, mild surge happens first, where the transient pressure signals show sinusoidal form with Helmholtz frequency of the compressor system. Reducing the mass flow rate, the mild surge vanishes and gets replaced by the local stall. Further reducing the mass flow rate, deep surge breaks out suddenly. During two-regime-surge, two distinct surge patterns exit and vastly narrow stable flow range, which highlights the characteristics of two-regime-surge. It is found that the impeller leading-edge stall is a necessary part of the mild surge, while the diffuser rotating stall incepts the deep surge. At higher speeds, the mild surge oscillation prompts the early occurrence of the diffuser stall so that the mild surge transforms and the deep surge happens in advance. As a result, both regimes of mild surge and deep surge are going to merge, and the stable flow range at high rotational speed is greatly narrowed. Impeller casing treatment is considered as an effective method for flow range extension because the impeller leading-edge stall is removed and the mild surge is avoided as well.