A characteristic variation of voltage rise in a low voltage level system occurs when cross-voltage ungrounded fault. In response to this problem, this paper establishes a model of cross-voltage ungrounded fault, based on which to explore the impact of various critical operating and design parameters on the overvoltage of low voltage level systems, and numerically evaluate the overvoltage multiples in the actual grid. The research shows that the overvoltage condition of the low voltage level system is the most serious when single-phase cross-voltage ungrounded fault. This overvoltage multiple is positively correlated with the high voltage level system power supply strength and negatively correlated with the low voltage system power supply strength; The erection position of the same tower has a great influence on the overvoltage multiple, and the mutual inductance between the lines has little influence on the overvoltage multiple. Based on the above rules, suggestions for the design of the same tower and the safe operation of the system are given.