In order to exploit horizontal time-harmonic electric dipoles to model shaft rate electric field of practical vessels, the magnitude and location of equivalent source need to be determined. Aimed at vessels mounting the impressed current cathode protection system, the fact that the equivalent source is superimposition of two directionally opposite horizontal time-harmonic electric dipoles is clarified by analyzing the underlying physical mechanism of shaft rate electric field. On the basis of the former conclusion, a practical calculation method of equivalent source magnitude of shaft rate electric field was proposed, which means that the magnitude can be obtained by combining source magnitude of static electric dipole inverted from measured static electric field with the ratio coefficient between the maximum value of shaft rate signal envelope and that of static electric field signal. Besides, the location of equivalent source is identical to that of static electric dipole. The effectiveness of the proposed method is validated by the vessel model test. 