The fault current in single-phase grounding is so small that it is very hard to detect, single-phase grounding protect problem have no very effectual solution as yet. According to fault line selection problem of small current grounding power system, the paper gives a new real-time monitoring and fault diagnosis device based on ARM S3C2440.It not only can really monitor system status, but also can measure some elementary power parameters, such as the three-phase voltages, current, power, frequency etc. Eventually, the design realize many function such as real time detection, fault phase selection, fault line selection, fault recording, device satisfies the practical requirement. Operation results show that this device computing speed quickly, strong anti-jamming capability, stable operation, is very application in the small current grounding.