Based on the traditional model transformation theory, and integrated with CFD numerical simulation technology, a new modelling design method of nuclear main pump impeller was investigated and proposed. The new method improves the traditional one, can strictly ensure compliance with design pressure, temperature, flow rate, speed, and obtain new impeller precisely corresponding with the design head. Compared with the traditional one, the new method is more accurate, dispensing with the repeated adjustment to the blade profile. It can get the new impeller according with design conditions only through one time transformation by choosing model impeller with similar specific-speed. The example proves that the new method has high applicability and design accuracy within a certain range.