The invention provides a system and a method for monitoring ground potential difference. The system comprises transformer substations and an alternating-current power transmission line, wherein transformer substation grounding flat steel is connected with a neutral point of a main transformer of each transformer substation; the main transformers of the two transformer substations are connected with each other through the alternating-current power transmission line; current flowing through the alternating-current power transmission line is generated by the ground potential difference; a wave recording device is connected between each piece of transformer substation grounding flat steel and the neutral point of the main transformer of the corresponding transformer substation; the wave recording devices are in communication connection with a GSM (global system for mobile communications)/ GPRS (general packet radio service) network; the GSM/GPRS network is in communication connection with a server of a monitoring master station; and each wave recording device is also connected with a GPS (global positioning system) synchronizer. A testing loop for monitoring the potential difference between the transformer substations is formed, a ground potential different monitoring network for covering a whole region is realized, a powerful basis is provided for analyzing a logical relationship between ground current activity and a power grid running mode, regional earthquake disaster monitoring and early warning platform establishment are improved, and a criterion is provided for abnormal running states of systems such as electric power systems and communication systems.