Standard k-ε turbulence model is used to simulate the fluid field and mixing characteristics in a tank stirred by a Smith turbine in a platform of commercial CFD software package，CFX10.0.The relative rotation between baffles and the impeller is numerically simulated by a Multiple Reference Frame approach.The numerical results indicate that when the impeller off-bottom clearance decreases from 1/2 to 1/3 of the impeller diameter，flow pattern is always the “typical two loops”; however，when the impeller off-bottom clearance decreases from 1/3 to 1/6 of the impeller diameter，flow pattern changes from the “typical two loops” into a “single loop”.Power number predicted by this work is in good agreement with design value and predicted value by other researchers.The snapshots of concentration distribution of tracer indicate that mixing time depends heavily on the flow field in the stirred tank.Both feeding and monitoring points have significant influence on the mixing results.It is important to choose appreciate monitoring points when collecting the mixing time data in practical industrial or experimental work.The above CFD simulation results indicate that the model employed in this paper can predict the mixing characteristics in the stirred tank with a Smith turbine，which provides helpful information for optimization and improvement in the future design of the Smith turbine.