The invention discloses a synchronization device for long air discharge observation. The synchronization device comprises an oscilloscope, an electro-optical conversion module, a first photoelectric conversion module, a second photoelectric conversion module, a current measuring module and a high-speed camera. The oscilloscope measures a to-be-tested voltage, outputs a trigger electrical signal according to a preset trigger level, and records transmission time of the trigger electrical signal. The electro-optical conversion module receives the trigger electrical signal outputted by the oscilloscope and converts the trigger electrical signal into a trigger optical signal. The first photoelectric conversion module and the second photoelectric conversion module receive the trigger optical signal, convert the trigger optical signal into a trigger electrical signal and trigger the current measuring module and the high-speed camera respectively. With the provided synchronization detecting device, high-precision synchronous measurement of the voltage, current and image of long air discharging can be realized; the test arrangement is simplified; and the test cost is saved.