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For analyzing the flow pattern and reducing the hydraulic loss, the jet wake flow in the channel of impeller and how it is influenced by the surface roughness have been studied by coupling computational fluid mechanics and the sand-grain roughness method. The results of experiment and simulation were compared to insure the RNG k-εmodel could reflect the variation of the inner flow. The results indicate that the jet wake flow was obvious in the channel of impeller and the jet flow mainly appeared nearby the rear shroud of the suction side, while the jet flow appeared in the front shroud at the exit of impeller. The surface roughness of the impeller had a great influence on the relative velocity, and had the opposite influence on the pressure and suction side. The effect of surface roughness on the relative velocity become opposite when the value of roughness was higher than a transitional point. With the increase of the value of surface roughness, the amplitude of the relative velocity at the exit of impeller increased obviously, which was caused by the impeller-volute interaction.