It is revealed that positive slope of head-flow performance curves appears when the pump is working at partial flow rates both for experimental and numerical investigations, and characteristics calculated agreed well with experimental ones. CFD was used to study the mechanism of the performance curve instability of a test pump. Because of the existence of high pressure differences at the partial flow rates, the fluid leakage occurred at the tip clearance between the casing and the impeller. Furthermore, pre-rotation generated at the inlet of the impeller. The theoretical head decreased because of the pre-rotation. When the flow rate decreased further, a strong vortex occured at the inlet of the diffuser hub and blocked the discharge of the impeller. Therefore the flow pattern of the impeller tends to a centrifugal type and the head of the mixed flow pump increased, subsequently the positive slope of performance curve occured. Two ways were taken to suppress the performance curve instability, the inadequate of the suppression method of the model and some positive results advices about the performance curve instability suppression are proposed. A swirl-stop and diffuser optimization could somewhat improve the instability but not ideal, while the proper shallow groove is a simple and useful method on suppression of pump performance curve instability.