The traditional theory of overvoltage of single-phase arc grounding is built on the basis of the strict symmetry of the capacitance parameters, because of the general existence of three-phase asymmetry of power systems, when the single-phase arc grounding fault occurs in the power grid, the actual overvoltage amplitude may exceed the amplitude limit calculated by the traditional overvoltage theory, which makes the power grid have the risk in insulation and overvoltage protection. In this paper, the physical process of single-phase arc grounding overvoltage is deduced theoretically, in view of the Peterson theory, the traditional overvoltage theory such as Peter and Slepian theory does not consider the three-phase asymmetrical degree of power system, this paper introduces three-phase asymmetrical variable, which makes the single-phase arc grounding overvoltage theory more accurate and more suitable to meet practical needs of the project. Finally, use the improved Mayr dynamic arc model to compares new theory and traditional theory in the simulation then makes the error analysis.