The effects of periodic suction on the separated flow are still unclear and the relevant researches are still scarce, thus this paper presents a periodic suction method to suppress the flow separation in diffuser. An evaluation method of periodic suction mass flow rate’s effect on total pressure loss is developed firstly and the comparison between periodic suction and steady suction’s effect on total pressure loss was performed. The result showed that compared to steady suction, periodic suction can provide a better control effect in a wide operating range, even nearly double in some control state. Besides, the effect of periodic suction on the frequency and phase of separation vortex was studied. It showed that periodic suction control method can achieve frequency and phase locking with very low control energy. What’s more, frequency and phase locking point corresponds to the turning point of the periodic suction exerting unsteady control’s advantage. The separation vortex and it’s relation with the pressure time histories and spectral density was also investigated. Lastly, the proper orthogonal decomposition was employed to further explore the control mechanism of periodic suction. The result showed that the main effect of the periodic suction is reallocating the energy of each mode, and strengthening the second and third modes, while the steady suction weakened these modes. It indicates that the reason of the control effect difference between steady and periodic suction may be that steady suction doesn’t effectively use the energy of vortex, but removed it directly. Also, compared to the steady suction state and no-control state, the time modal coefficients in the first three modes are most orderly in the periodic suction state.

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