In low-voltage distribution system 3rd harmonic current content is big enough. It is a common characteristics. Through analyzing the characteristics of harmonic, a new open-neutral fault detection approach is proposed that monitoring device can be simply installed at feeder head. Main idea of the proposed approach is that the fault can be identified according to the 3rd harmonic variation measured at the feeder head. Simulation and experimental analysis shows that the 3rd harmonic current will show varying degrees of change at the feeder head when open neutral fault occurs, regardless of the load is balanced or unbalanced. The fault location is closer to feeder head indicates that the 3rd harmonic current change is more significant. This paper researches and analyzes the 3rd harmonic and fundamental currents sudden change method at feeder head when open-neutral fault happened, as well as 3rd harmonic and fundamental currents of load fluctuation method during normal operation. It obtains fault criterion and fault identification processes that can effectively distinguish between open-neutral fault and normal operation. The experimentally measured load fluctuation data for simulation study verifies the fault detection criterion is validity to open-neutral fault whenever in balanced and unbalanced load conditions.