Electrostatic field can modify transfer the physical properties of dielectric system, and the related researches and applications attracted more and more attention. This paper discussed in detail the actions of uniform electrostatic field on the pressure and mole fractions of a fluid dielectric system with a model of a single-phase fluid dielectric system with a parallel plate capacitor inside. Based on the principle of phase equilibrium, the variations of pressure and mole fractions of gas and liquid dielectric system with electric field intensity were derived by using the expression of chemical potential with the influence of electric field. The results show that the changes in pressure and mole fraction are not only proportional to the square of electric field strength, but also related to the temperature and the molar mass, density, and permittivity of the dielectric. The actions of electrostatic field can increase the pressure of the system and change the mole fraction of the mixture. However, whether the mole fraction of the component increases or decreases is determined by the relative magnitude of the molar mass and density of the components. Under the actions of electrostatic field, variation of liquid system is more significant than that of gas system. The findings of this research provide a new idea for measuring the dielectric constant, gas enrichment and composition controlling of mixture.