The invention relates to a portable fault oscilloscope mounted in a live-line mode. The portable fault oscilloscope comprises an independent power source, a sensor plug-in, a switching value module, a GPS time check module, an ADSP plug-in, a CPU plug-in and a temperature/humidity sensor module, wherein the independent power source provides a rated power source voltage for each plug-in, the sensor plug-in converts a voltage and a current into signals which can be received by the oscilloscope internally, a loop current signal is acquired in a live-line mode, the switching value module receives a switching value signal inputted by an electric power system, input output isolation for the switching value signal is carried out, and a safety switch signal is formed; the GPS time check module employs hardware pulse and IGIR-B code time check modes to realize oscilloscope and GPS clock synchronization, the ADSP plug-in receives a simulation voltage signal and a simulation current signal and receives the safety switch signal provided by the switching value module, the CPU plug-in receives oscillograph data sent by the ADSP plug-in, and storage, communication and analysis of the oscillograph data are accomplished.