The invention relates to a fault determining system and method used for a cable and aerial conductor hybrid line. The fault determining system comprises two operation current and fault current multiplexing sensors, current gatherers and one fault analyzer, wherein the two operation current and fault current multiplexing sensors are respectively arranged at connecting positions between the two ends of a cable and an outdoor terminal, the operation current and fault current multiplexing sensors are each connected with one current gatherer, and the current gatherers are connected with the fault analyzer. The operation current and fault current multiplexing sensors respectively acquire currents flowing through the two ends of the cable, the current gatherers obtain operation currents and fault currents for sending to the fault analyzer, and the fault analyzer, according to the size and the polarity of the fault currents and waveforms of the operation currents within a set time period before and after the time when the fault currents are generated, makes an analysis so as to carry out fault determination. Compared to the prior art, the system and method provided by the invention have the advantages of simple structure, high efficiency, high power supply reliability and the like.