The invention provides a power grid line fault diagnosis system and method based on a Hadoop cloud computing system. The system comprises a voltage transformer, a current transformer, a transmitter, a remote terminal, a modem, an optical fiber and a plurality of upper computers. The method includes the steps that voltage and current of a power grid line are acquired in real time and transmitted to the remote terminal; when a power grid has a fault, electrical quantity information of the power grid is acquired and transmitted to a NameNode in a Hadoop cluster and then is assigned to a DateNode to be processed in parallel, and the fault of the power grid is diagnosed and displayed. According to the power grid line fault diagnosis system and method, the electrical quantity information of the power grid line is fully utilized, the current, the wavelet signal rate of the voltage, the wavelet fault rate and the wavelet variation rate are calculated according to the result obtained after wavelet transform reconstruction based on multi-resolution analysis, fusion is performed to form a preliminary credibility distribution function, accurate division is performed after preliminary classification is performed on a fault power grid line set and a non-fault power grid line set, the fault power grid line set is determined finally, and fault power grid lines can be accurately judged out.