Yellow phosphorus is an important industrial raw material for the development of phosphorus chemical industry in China. The yellow phosphorus production by electric-furnace method has always been the core of thermal process for phosphate rock treatment. The electric-furnace method should be able to make direct use of the low-grade phosphate rocks, especially those with high silicon content, but this would consume a lot of energy. The researchers note that attention should be given to the formation of eutectic and the adjustment of eutectic temperature. A large number of experimental studies are obtained to discuss the macroscopic effect of different strengthening means of phosphate rock reduction. However, the mechanism is mostly inferred. At present, the lack of objective presentation of the eutectic composition and transformation hinders the clear explanation of those mechanisms. Thus, it is necessary to study the phase diagram of slag and the melting aid effect in the phosphorus production process by means of chemical thermodynamics computation. It is of great realistic and scientific significance to reveal the interaction mechanism and improve an electric-furnace technology of phosphorus production. In this work, the phase diagram and melting mechanism in the phosphorus production process by electric-furnace method were studied with FactSage 8.0 computation. It showed that levels of Al2O3 had a certain effect on the formation of eutectic and the adjustment of eutectic temperature. With the proper increase of Al2O3 content, the phosphorus production process was improved. In contrast, the single MgO as a flux was unable to effectively promote the charge melting and the eutectic temperatures were likely to rise. Beyond these points, the melting reduction reaction of the phosphate rock was demonstrated to be consistent with the first-order reaction characteristics.