Abstract: Vibration has been an ever-increasing demand for the operational stability of centrifugal pumps in recent years, especially in most large pumps in modern industry. The internal flows are generally dominated by the characteristics of operation stability and vibration of centrifugal pumps. Nevertheless, the blade and volute structures are dramatically detrimental to the internal flows. The radial force is one of the most significant factors to affect the operation stability, and the pressure pulsation inside the volute, particularly in the tongue areas. A double-volute structure is selected normally to reduce the radial force in commonly-used large pumps, instead of a single-volute structure. But the specific structures are still required to be optimized, such as the arrangement of the rib start point. In this study, a three-dimensional unsteady dynamic was proposed to clarify the influences of rib start point on the hydraulic performance, radial force, and pressure fluctuation in the volute of a large double-volute double-suction pump. The variation of fluctuation amplitude and frequency of radial forces were considered at the different rib start points. Meanwhile, the radial forces of the double-volute pump at various flow rates were also compared with those of the single-volute pump. The experiments of pump performance (head, efficiency, and power) were performed on the large-scale pump platform in KaiQuan Factory in Shanghai of China. The numerical data agreed well with the experiments, same as the grid independence check. The results indicated that the hydraulic efficiency at the design points decreased about 4%-5% in the double-volute structure, compared with the single-volute structure. Periodic variation of radial force was detected in the test, where the blade passing frequency was dominated during a blade revolution. The angle of radial force was also changed periodically. The rib start point significantly determined the radial force, where the minimum appeared when the rib started from 190° and 200°, whereas, the maximum appeared when the rib started from 212°. Seven monitoring points were located in the tongue areas to collect the pressure fluctuations. It was found that all the mean, peak, and fluctuation values at the points behind the tongue were much less than those in the front of the tongue under the different rib structures, only 25%-50% of the corresponding value of points in the front of the tongue. The pressure fluctuations of points in front of the tongue were dominated by the blade passing frequency, whereas, those points behind the tongue were dominated by the double blade passing frequency. More importantly, the pressure fluctuation of points declined dramatically, when the rib start point moved backwards, indicating a cause of double blade passing frequency. Thus, the rib start point should be placed at 200° from the tongue point in the flow direction, where the maximum efficiency and the minimum radial force can be achieved, particularly considering the coupled interaction of hydraulic performance and radial force. The finding can greatly contribute to the hydraulic improvement and structure optimization in large centrifugal pumps. Key words: pumps; experiments; rib location; double-volute pump; radial force; pressure fluctuation