Variable Nozzle Turbine (VNT) could change the flow area by adjusting the angle of nozzle blade and the mass flow rate and efficiency characteristic to match turbine and engine well in a extensive operating range. By the use of 3D numeric simulation technique, the numerical calculation and flow field analysis on incidence of impeller of JK90S variable nozzle turbine were carried out. As for impeller incidence, the incidence corresponding to best efficiency is different under different throttling and engine speed. At 100% throttling of engine, the incidence of best efficiency is about -45°; at 50% throttling of engine, the incidence is about -35°. The efficiency of impeller decreases rapidly when the incidence is less than optimum; when incidence is larger than best efficiency incidence, it could maintain high impeller efficiency during an extensive range. With the engine speed increases, the incidence range that corresponding to high efficiency of impeller eventually reduces.