Abstract: In order to study the inner flow mechanism of the fluid-structure interaction (FSI) effect on the external characteristics, the external characteristics and internal flow features of a diffuser pump were analyzed with the two-way flow solid coupling method. The steady flow field was based on Reynolds Averaged N-S equations with standard k-E turbulent model, the unsteady flow field was based on the large eddy simulation, and the structure response was based on elastic transient structural dynamic equation. Compared with the results without FSI, the total pressure and velocity distributions at the outlet of the impeller were analyzed. Inner flow mechanism for the effect of the FSI on prediction of the external characteristics was calculated. And the pressure fluctuations of the impeller at outlet were also analyzed. The results show that, the main reason for the variation of the external characteristics prediction based on the effect of the FSI is the deformation of the impeller, especially at the outlet of the impeller. Considering the effect of the FSI, the tendency of the pressure fluctuations is similar to the results without FSI, but the amplitude is larger. Maximum pressure pulse amplitude is improved by 3.1 percent. The results can provide reference for the research of performance prediction and flow induced vibration in centrifugal pump.