The invention relates to a single-phase earth fault positioning method and a system for a self-adaptive mix-earth mode. According to the invention, firstly, the real-time earth mode of a system is determined, and then a fault judging method is selected according to the earth mode. If the earth mode is realized through earthing a small resistor, a fault section is determined by adopting a waveform similarity method. If the earth mode is realized through earthing a small current, the fault section is determined by adopting an amplitude comparison method. According to the invention, the system can be automatically applied to the change of the earth mode, so that a single-phase earth fault can be positioned. The reliability of power supply is improved.