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It is vexatious for conventional methods to calculate the compositions of equilibrium system with many components and minor components. For that, the element potential method (EPM) for composition calculation in multi-phase equilibrium system was deducted based on the principle of Gibbs free energy minimization and the law of mass conservation, by introducing Lagrange factor lambdae and the concept of element potential. Equilibrium composition of some complex systems, such as methanol synthesis, ammonia oxidation and aniline-hexane system were calculated using EPM. Results show that, compared with the equilibrium constant method and other conventional methods of free energy minimization method, EPM can acquire rapidly accurate results with less variables,and there are no negative mole fractions appear during the computational process. Therefore, EPM is feasible and effective to calculate equilibrium composition of multi-phase multi-component system,especially for that one with minor components, and can be applied widely to metallurgical and chemical fields.