Abstract: The core of seawater circulating pump motor in the nuclear power plant is responsible for providing the path for the establishment magnetic flux, core design needs to consider magnetic density, heat load, ventilation and other factors. The core temperature rise is lower than the limit under the condition that the magnetic saturation of each core part does not occur. The magnetic circuit calculation, ventilation calculation and core development are introduced, and the iron loss test of stator core is carried out, It accumulates experience for nuclear power plant large cycle pump motor core design manufacture and test.