Due to the application of insulated wire, the occurrence probability of line break faults in distribution lines increases. There are no effective method to detect line break fault on site. In this paper, based on ungrounding system, a model of single-phase line break fault without grounding is established. Line break location, the ratio of the fault line grounding capacitance to system grounding capacitance, load impedance magnitude and distribution factors are taken into consideration. Variation characteristics of offset voltage at neutral point, line voltage and phase voltage of the line break point are analyzed. According to the variation characteristics of line voltage and phase voltage, this paper proposes fault section location methods respectively to detect single-phase line break fault without grounding. This proposed method can be realized through distribution automation platform or malfunction indicator system, which improves the reliability and applicability of line break fault section location. In the end, this method is verified by Matlab simulation. This work is supported by National Natural Science Foundation of China (No. 51477184).