The performance deficiency of non-overload centrifugal pumps was analyzed. A design idea of non-overload centrifugal pumps was suggested that combined the enlarged flow design with the clogged flow passage design. And, three improved attempts on IS50-32-160 non-overload centrifugal pump were carried out supported by commerce software Fluent to forecast the performance of centrifugal pumps. The forecast results indicate that by adopting this design thought, the head and efficiency of non-overload centrifugal pumps is increased. Furthermore, a reference is provided for optimal design of low specific speed centrifugal pump.