The utility model provides a fault arc detecting device, include: analog to digital converter ADC, digital signal processor DSP and microcontroller MCU, this ADC is used for from direct current system received voltage signal to send this voltage signal to DSP, this DSP be used for according to this voltage signal obtain the interval harmonic content of a preset frequency with to according to this harmonic content with judge whether this direct current system takes place DC fault electric arc, and send the judged result to MCU. Through the utility model discloses a detect the fault electric arc among the direct current system, before electric arc further produces harm or even arouses the conflagration, send alarm signal and in time cut off the direct current system circuit, protection direct current system.