In order to study the impeller internal flow field of the mixed-flow pump with nonuniform tip clearance, the internal flow field in mixedflow pump with two eccentricities which were 0, 0.3mm and 0.5mm was numerically simulated based on the standard RNG k—ε turbulence model and SIMPLE algorithm. Besides, the external characteristic, static pressure distribution on blade surface, tangential pressure, turbulent kinetic energy distribution and streamline distribution near the tip clearance of mixed-flow pump were compared under the conditions with or without eccentricity. The results showed that the type of numerical simulation grid and the turbulence model can accurately predict the internal flow characteristics of the mixed-flow pump. The maximum decrease of head in mixed-flow pump was 9.8%, the efficiency at the design point in mixed-flow pump was dropped by 4.3% and the high efficient point was swung to the large flow rate when eccentricity e was 0.5mm. The pressure distribution of inlet and outlet of mixedflow pump impeller was affected greatly by eccentric, and pressure distribution near the eccentric side of the blade outlet flange showed a trend which was gradient distribution along the radial and circumferential pressure distribution was greatly imbalanced. Non-uniform tip clearance seriously interfered with the end wall region flow, which led to the unsteady flow phenomenon of the leakage flow and the secondary flow in the flow field of the tip clearance was obviously increased, the turbulent kinetic energy dissipation was increased with the increase of eccentricity and hydraulic loss, which was the main factor resulting in a decline in the efficiency of mixed-flow pump. The research results had reference meaning for revealing the internal flow characteristics of impeller in mixed-flow pump with non-uniform tip clearance.