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Aiming at the problem of reverse flow which occurred under low flow rate in a low specific speed double suction centrifugal pump, the inner flow of pump and suction chambers were simulated, velocity analysis of fixed-points which was proposed to simulate probe-sampling system in experiment was used in reverse-flow research. After the statistics and analysis of velocity changes of fixed-point in the inlet of impeller in different flow rates, it was found that the curve got from simulation accord with the theory analysis and experimental curve in other literature. Based on the analysis of curve, reverse occur flow rate, reverse stable flow rate and reverse flow range were defined and explained. A new method which was used to predict the effective thickness of reverse flow field according to theory analysis was proposed.