Abstract: View of the frequent occurrence of abnormal electric brake cut off of one unit or the entire train during A5 type vehicle operation of Guangzhou Rail Transit Line 2, in-depth investigation and cause analysis of abnormal electric brake cut off problems are carried out. It is found that back electromotive force emerges at the moment of contactor breaking circuit, in which cases the gateway valve might mistakenly detect high level voltage and judge that train is entering emergency traction mode, causing the electric brake to be abnormally cut off. In response to the fault cause, solution of controlling contractor electricity detecting point by reforming air-compressor is proposed. The upgraded A5 type vehicle is functioning well, having the problem of abnormal electric brake cut off solved.