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Numerical simulations based on CFD had been performed by the IS150—125—315 singl-stage single-suction centrifugal pump. The numerical results showed a good agreement with the experimental measurements. The performance, i.e., head, efficiency and shaft power of pump were studied under five balance hole diameters. For each of the diameter, the spatial distribution law of balance cavity pressure was researched. The influence of holes diameter on shroud force was analyzed and the relation curve p-=f(k-) was further drew. The results showed that the head had no relation with balance hole diameter when the flow rate was larger than design flow condition. After balance hole diameter increased to a certain value, shaft power was obviously increased and the efficiency was reduced at the same flow condition. The differential pressure of centrifugal pump without balance holes is larger than that with balance holes at the same massflow. Balance cavity pressure of the holes with the same diameter increased along radial direction from pump shaft to seal ring, while the change of its value was negligible along axial and tangential directions. In the region where the radius was less than the vertical distance between balance holes center and pump shaft center, a smaller diameter of balance cavity made the pressure approach zero. Out of this radius, the pressure increased as the balance hole diameter increased. When k-≥2.645, the shroud force on the balance cavity almost achieved balance.