The present invention discloses a transformer DC monitoring system, which consists of a far-end measurement control unit, an optical fiber channel input unit, an optical fiber channel, a server, a workstation and a client. A prepositive computer is arranged between the optical fiber channel input unit and the optical fiber channel; the prepositive computer consists of a main prepositive computer, a deputy prepositive computer and a dual-computer switching device; the dual-computer switching device is connected with the optical fiber channel input unit; the main prepositive computer and the deputy prepositive computer are both connected with the dual-computer switching device; the communication mode between the main prepositive computer and the deputy prepositive computer adopts GPS wireless mode; data monitoring, parameter setting, data analog, data pre-treatment and data transmission computer programs are arranged in the main prepositive computer and the deputy prepositive computer in advance. The transformer DC monitoring system makes full use of existing equipment to establish a long-distance centralized DC monitoring system which is applicable to various substations and improve the comprehensive monitoring level, openness and expandability of the system and the stability and reliability of the monitoring. The transformer DC monitoring system is used for power systems.