The invention discloses a real-time online measurement system and a real-time online measurement method for the resistance of a ground wire. The measurement system comprises a microprocessor module, an interference source processing module, a communication module, a ground wire resistance measurement software module, a power management module, a constant current source injection module, a sampling voltage reading module and an interference source acquisition module, wherein the sampling voltage reading module measures sampling voltage under the control of the microprocessor module; and the ground wire resistance measurement software module calculates the resistance of the ground wire according to the sampling voltage measured by the sampling voltage reading module under the control of the microprocessor module. The resistance of the ground wire at a single point or a plurality of points can be measured on line in real time, the system is arranged at one time, and is time-saving and labor-saving, and high accuracy is maintained; the resistance of an auxiliary current electrode and the voltage of an interference source can be measured, the system and the method also have a real-time online function, data can be transmitted to a monitoring center for monitoring, and an alarm can be given according to settings; and in addition, the system and the method have a function of diagnosing the disconnection of an external connecting wire.