The numerical calculation about the interior flow field of the screw centrifugal pump with the CFD software by changing the clearance between the impeller outer edge and shell was given and based on which the exterior performance of the pump was predicted. The performance experiments were done at the same conditions. Compared the performance prediction with the experimental results, the effect of clearance on the pump performance was analyzed and the reasonable range of clearance was given which provided some meaningful references for the design of screw centrifugal pump, and the importance of the numeric simulation method was indicated.