With the development of novel combined-field separation techniques, the influence of external fields on the thermodynamic properties of working systems is an important area of study. A study on the phase equilibrium of n-butanol/citric acid/water under an electric field was carried out. The effects of field strength, temperature, and solute concentration in the aqueous phase on the distribution coefficients were investigated, and the results were analyzed through chemical potential equations. The experimental results show that, if enough time is allowed, an equilibrium state will be reached for the working system under the action of an electric field. Increased voltages and temperatures resulted in higher distribution coefficients between butanol and water. If the solute concentration in the aqueous phase is lowered, the distribution coefficient is strongly increased with an increase of the field strength. However, at unchanged field strength, a higher solute concentration will cause a decrease in the distribution coefficient. Also, the logarithm of the ratio defined by the distribution coefficient under an electric field to the distribution coefficient without an electric field is directly proportional to the voltage applied to the system.