It is a long-standing issue on fault line detection of single-phase grounding fault in the small current grounding systems. If only one fault line detection method is used, fault information will be analyzed partially, which is not enough for fault line detection; and there are different fit conditions for every method. So the single method can not ensure that the reliability of the fault line detection. In this paper, the effective domains of fault line detection through some methods were obtained by rough set theory, and the radial basis function (RBF) neural network was designed and trained, then the results of the methods based on RBF network were got. Fusing those detection results, a better fault line detection result was advanced. Simulation results by EMTP show that the fault line detection method is efficient with high value of studying and wide application future.