Abstract: Pantograph current-collecting equipment is an important equipment for trains to introduce external power into the vehicle power supply system. If failure occurs on main line, operation will be affected. When the train of Suzhou Rail Transit Line 5 was put into operation for the first time, an abnormal single bow descending fault occurred before train leaving the depot and going online. The fault phenomenon is introduced, with the schematic diagram of pantograph bow ascending and descending control, the reason why the pantograph abnormally lowers single bow is analyzed. This fault is caused by the reverse connection of the ascending bow train line and the descending bow train line and occurs before the delivery of the vehicle. Therefore, it is suggested that all test items should be completed before delivery.