Aiming at diagnosing the eccentricity fault of interior permanent magnet synchronous motors (IPMSMs), this paper analytically calculates the radial component of the open-circuit external field of a typical traction IPMSM under the eccentricity fault. Firstly, the radial component of the open-circuit air-gap field is calculated by an Equivalent Magnetic Circuit model considering the temporal and spatial distribution of the changing air-gap length. Then, the radial component of the open-circuit external field is calculated based on improved attenuation coefficients. Finally, the pattern of how the eccentricity fault impacts the open-circuit external field is analyzed. It is found that the open-circuit external field is sensitive to the eccentricity fault and thus is capable to be adopted as a diagnostic signal of that fault.