In order to reveal the dehydration mechanism under the coupling of an electric field and a magnetic field, the dehydration rate of water-in-oil emulsions was tested after the treatment by a rapid evaluation experiment system, where an electric field and a magnetic field could be generated synchronously. The main factors that cause the coupling action were analyzed quantitatively. The results showed that the single steady magnetic field had no obvious effect on dehydration of emulsions. However, the dehydration rate of emulsions in the coupling field of the ac electric field and the steady magnetic field was superior to that of the single ac electric field, and the increase of particle size of the dispersed phase was conducive to improving the dehydration efficiency. For the coupling effect of the dc electric field and the steady magnetic field, the dehydration rate had no obvious improvement compared with the single dc electric field. Moreover, the calculation results showed that the polarization magnetism force generated by transient polarization was the main factor for the efficient separation of oil and water under the coupling field of the ac electric field and the steady magnetic field.