An online monitoring system for monitoring distribution of DC (direct current) in an AC (alternating-current) network comprises Hall current sensors (4), a DC monitoring controller (6), and a low-pass filter (7) and an A/D converter (8) which are used for preprocessing a DC monitoring signal. The online monitoring system is characterized by further comprising a GPS synchronous signal module (9) which is used in a synchronous DC signal sampling moment, and a wireless network transmission module (10) which is used for transmitting the DC data and the sampling moment to a central server database module (11); and the online monitoring system is further characterized in that the Hall current sensors (4) are mounted on a lightning conductor (3) over a tower (1) and at a neutral point of a transformer (4). The online monitoring system can simulate and calculate the distribution of DC in the AC network, and meanwhile monitor the DC flowing through the neutral point of the transformer and the lightning conductor, and therefore can economically and comprehensively reflect the distribution of the DC in the AC network and accurately determine a DC magnetic biasing level of the transformer.