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The unsteady numerical simulation of centrifugal pump using SST model was conducted under different relative installation positions of guide vane and tongue. The errors of efficiency and head between test and simulation were within 5%. Investigation results showed that when the guide vane and volute tongue installation angle α was 25°, the head and efficiency were higher than those at other installation positions under various operating conditions. The minimal efficiency and head were occurred at α of 5°. When the flow rate Q was 24 m 3/h and 32 m 3 /h, the maximum and minimum efficiencies differences were 1% and 1.4%, respectively; when Q was 40 m 3/h, 48 m 3/h and 56 m 3 /h, the differences of the maximum and minimum efficiencies were 2.2%, 2.4% and 4.5%, respectively. The velocity at impeller outlet changed with different time steps, which caused the radial force vectors acting on the impeller appeared much irregular distribution. The influence of different relative installation positions of guide vane and tongue on the radial force was mainly attributed to its influence on the static pressure at impeller outlet which led to asymmetric output of static pressure, and affected the impeller radial force. When α was 41° and 25°, the symmetry of the impeller outlet static pressure was significantly better than those of other relative positions, and the radial force was smaller than those at α of 5° and 59°. When volute tongue apart gradually closed to guide vane, the efficient and head of pump were lower. The highest efficient and head occurred when volute tongue was in middle of two vane, which can be a guidance to engineering practice.