Because current choose accuracy of the NUGS choosing earth-fault line device is not high and the range of the choosing line is limited.A new criterion is advanced according to the analysis of the NUGS single-phase grounding characteristic and various methods of choosing earth-fault line.This criterion uses the zero sequence power method（the most efficient method among all stable-state methods） as the main criterion

and the fifth harmonic phase method as the second criterion.A single-phase grounding fault of a 110/10 kV power station is simulated by using ATP simulation software.The results show that

to different fault position and different grounding resistance

the criterion can find out the fault line exactly.Thus a new device is designed based on the PC104 industrial computer.