The invention provides an ultrahigh-voltage DC fault diagnosis system based on a model and a rule base and a working method thereof. The system and the method belong to the field of power system intelligent analysis and controlling technology. Currently, development of an ultrahigh-voltage AC/DC and multiple-DC power grid bring high challenge to safe and stable operation of the power grid. According to the system, a DC model is established in a local intelligent power grid dispatching control system, and fault diagnosis is performed on the ultrahigh-voltage DC current according to a real-timedisplacement signal, a protecting signal and remote measuring data based on a rule base. The diagnosis types comprise converting rheological fault diagnosis, valve set fault diagnosis, DC single-poleblocking fault diagnosis, DC double-pole blocking fault diagnosis, AC filter fault diagnosis, grounding DC current overlarge diagnosis and commutation fault diagnosis. Furthermore a DC operation stateis monitored. A diagnosis result and an operation state are alarmed in the workstation of the dispatching control system in a picture form. The ultrahigh-voltage DC fault diagnosis system and the working method have advantages of perfecting ultrahigh-voltage DC real-time operation monitoring facilities, and improving ultrahigh-voltage DC fault disposal capability of operation regulating personnel.