The invention relates to an impeller optimized design method capable of prolonging running down time of a reactor coolant pump. According to the impeller optimized design method, firstly, a main objective function for improving running efficiency under a running down working condition by optimizing main parameters of an impeller is established; then the main parameters are divided into three groups which are taken as design variables respectively, three subordinate objective functions are optimized respectively with a complex method for running down efficiency under initial constraint conditions, and the optimum points, guaranteeing the highest efficiency, of main geometric parameters of the impeller are obtained respectively; and a new constraint is determined in a small range nearby the optimum points, optimized solution is performed on the main objective function under the reduced new constraint with a feasible direction method, and the final optimized result is obtained. By means of design of the impeller of the reactor coolant pump, the problem of lower running down efficiency of the impeller can be improved, the running down time is prolonged, and the nuclear safety is improved. Meanwhile, with innovative application of the step-by-step optimization design method adopting constraint range reduction for subordinate objectives and optimum point searching for the main objective, the optimization speed is increased, and the effectiveness of the optimized result is guaranteed.