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In order to investigate the complex flow characteristics inside an unshrouded centrifugal impeller with splitter blades at off-design conditions, and analyze its influence on pump operation stability, a numerical simulation study was carried on using the curvature-corrected SST-CC turbulence model;the head and efficiency accorded with experimental results. The pressure fluctuation, unsteady radial force and velocity were analyzed quantitatively and the numerical results indicate this:the peak to peak value of pressure fluctuation in the impeller channel gradually increases in the flow direction and at 0.49Qn, the slope of peak to peak value to normalized impeller channel behind the splitter blade is 8.57 times greater than that before the splitter blade. The greater the flow rate deviates from the design condition, the larger the peak to peak value of the pressure fluctuation and radial force;in particular at 0.27Qn, the maximum radial force is 194.29％greater than that of the design condition. When the operating flow rate is smaller than 0.83Qn, the stall occurs and the stall vortex could block the impeller discharge;as the flow rate decreases further, the pressure amplitude at rotational frequency gradually increases in the impeller channel and the prevailing frequency changes from the blade passage frequency (BPF) to the rotating stall frequency in the diffuser. The tip leakage vortex (TLV) is generated in the tip region and rotated move downstream in the impeller flow channel, and the backflows appear on the blade suction side and in the tip and the tongue regions;the smaller the flow rate, the more serious the TLV and backflow phenomenon. The rotating stall causes uneven flow in the impeller channel, increasing the pressure fluctuation and the radial force, and resulting in an imbalance of the impeller rotation.