The distribution networks in china are mostly neutral point ineffectively grounded power system. It is of great significance to select the fault line quickly after single-phase grounding faults. By analyzing small current neutral grounded system’s equivalent zero-sequence network, the differential equations of zero-sequence current and zero-sequence bus voltage is established. Furthermore circuit’s earth capacitance is transformed into equivalent line length. Then use strong tracking filter to estimate equivalent length of each line, which can identify the fault line. The method has been verified via matalab simulations. Besides it can select fault line successfully with different fault condition, including different initial fault angle, different transition resistance, different fault position. In particular, it can be combined with feeder protection to meet the requirement for automation.